A real estate investment fund recently defaulted on $750 million of mortgages for two Los Angeles skyscrapers. A private equity firm slashed the value of its investment in the Willis Tower in Chicago by nearly a third. And a big New York landlord is trying to extend the deadline for paying down a loan for a Park Avenue office tower.

Office districts in nearly every U.S. city have been under great stress since the pandemic emptied workplaces and made working from home common. But in recent months, the crisis has entered a tense phase that could damage local economies and cause financial hits to real estate investors and scores of banks.

Lenders are increasingly reluctant to make new loans to owners of office buildings, especially after the collapse of two banks last month.

“They don’t want to make new office building loans because they don’t want more exposure,” said Scott Rechler, a New York landlord who is a big player in the city’s office market and sits on the board of the Federal Reserve Bank of New York.

The timing of the pullback in lending couldn’t be worse. Landlords need to refinance about $137 billion of office mortgages this year and nearly half a trillion dollars in the following four years, according to Trepp, a commercial real estate data firm. The Federal Reserve’s campaign to fight inflation by raising interest rates has also substantially raised the cost of loans still on offer.

Banks’ unwillingness to lend and building owners’ desperation for credit have created a standoff. Lenders want to extend loans and make new ones only if they can get better terms. Many landlords are pushing back, and some are threatening to default, effectively betting that banks and investors stand to lose more in a foreclosure.

How private negotiations between lenders and building owners are resolved could have major ramifications. Defaults could heap pressure on regional banks and help push the economy into recession. Local property tax revenue, already under pressure, could plummet, forcing governments to cut services or lay off workers.

“What we are seeing is this dance between lenders and owners,” said Joshua Zegen of Madison Realty Capital in New York, a firm that specializes in financing for commercial real estate projects. “No one knows what the right value is. No one wants to take a building back,” he said, adding that building owners don’t want to put in new capital, either.

He added that the office sector was feeling far more stress than other kinds of commercial real estate like hotels and apartment buildings.

Some industry experts are optimistic that given enough time, building owners and their lenders will hammer out compromises, avoiding foreclosures or a big loss in property tax revenue because everybody wants to minimize losses.

“I don’t see it as something that is going to result in systematic risk,” said Manus Clancy, a senior managing director at Trepp. “It’s not going to bring down banks, but you could see some banks that have problems. Nothing gets resolved quickly in this market.”

Loans on commercial buildings are typically easier than home mortgages to extend or modify. Negotiations are handled by bank executives or specialized finance firms called servicers, which act on behalf of investors that own securities backed by one or more commercial mortgages.

But striking a deal can still be hard.

Mr. Rechler’s company, RXR, recently stopped making payments on a loan it used to finance the purchase of 61 Broadway in downtown Manhattan. His company got its original investment in the building back after selling nearly half its stake to another investor several years ago, he said. He added that the lender, Aareal Bank, a German institution, was considering selling the loan and the building.

“In this illiquid market, can they sell that loan? Can they sell the building?” Mr. Rechler said. Aareal Bank declined to comment.

Eric Gural is a co-chief executive of GFP Real Estate, a family-owned firm that has stakes in several Manhattan office buildings, mostly older ones. He has been embroiled in nearly seven months of negotiations with a bank to extend a $30 million loan on a building in Union Square, and just two months are left on the mortgage.

“I’m trying to get a one-year extension on an existing loan so I can see what interest rates look like next year, which is likely to be better than they are now,” Mr. Gural said. “Hybrid work has created fear in the banks.”

Though many workers have returned to offices at least a few days a week, 18.6 percent of U.S. office space is available for rent, according to Cushman & Wakefield, a commercial real estate services firm, the most since it started measuring vacancies in 1995.

Public pension funds, insurance companies and mutual fund firms that invest in bonds backed by commercial mortgages also have an interest in seeing problems resolved or put off. A wave of foreclosures would lower the value of their securities.

Many of the mortgages that analysts are most worried about involve buildings in Chicago, Los Angeles, New York, San Francisco and Washington — cities where there is a glut of vacant space or where workers are reluctant to return to offices.

One such property is the 108-story Willis Tower in Chicago — the third-tallest building in the country, after One World Trade Center and Central Park Tower, both in Manhattan. The giant private equity firm Blackstone bought it for about $1.3 billion in 2015 and committed to spending $500 million on renovating the 50-year-old building, formerly the Sears Tower, including adding retail space and a rooftop terrace.

But in December, United Airlines, the building’s largest tenant, paid an early termination fee and vacated three floors; the company still occupies 16 floors. That month, about 83 percent of the building was occupied, according to KBRA Analytics, a credit data and research firm. Blackstone disputes those numbers; Jeffrey Kauth, a company spokesman, said that “approximately 90 percent of the office space is leased.”

Blackstone recently notified some of its real estate fund investors that it had written down the value of its equity investment in Willis Tower by $119 million, or 29 percent, said a person briefed on the matter, who spoke on the condition of anonymity to discuss sensitive financial information.

In March, Blackstone got a fourth extension on the $1.33 billion mortgage, pushing the due date to next year, according to Trepp. Under the terms of the loan, the firm can seek another one-year extension next year.

Blackstone said only around 2 percent of the firm’s real estate funds were invested in office buildings — down a lot from a decade ago.

Even streets with some of the priciest real estate in the country are not immune.

In Manhattan, the owner of 300 Park Avenue, an office building across the street from the Waldorf Astoria, is seeking a two-year extension on a $485 million loan coming due in August, according to KBRA Analytics. The property is owned by a joint venture including Tishman Speyer and several unnamed investors.

The 25-story building, built in 1955, is the headquarters for Colgate-Palmolive. But the consumer products conglomerate is shrinking its presence there.

“We requested that our loan be transferred to the special servicer well in advance of its maturity so that we can work together on a mutually beneficial extension,” said Bud Perrone, a spokesman for Tishman Speyer.

Portions of a bond deal that includes the 300 Park Avenue loan were downgraded last fall by Fitch Ratings because some tenants had left the building, and a lower-rated slice of the bond now trades at about 85 cents on the dollar.

Across the country, an investment fund connected to the real estate giant Brookfield Properties defaulted on $750 million of loans for the Gas Company Tower and a nearby building, 777 Tower, in downtown Los Angeles, setting up a possible foreclosure or a sale of the properties, according to the fund.

Andrew Brent, a spokesman for Brookfield, said in an emailed statement that office buildings suffering financial challenges were “a very small percentage of our portfolio.”

Even as building owners struggle with vacancies and high interest rates, some have found a way to put their properties on a more solid footing.

The owners of the Seagram Building at 375 Park Avenue in Manhattan have been working to refinance a $200 million portion of a loan that comes due in May while finding new tenants to fill several floors previously occupied by Wells Fargo.

RFR Holding, an investment group led by Aby J. Rosen and Michael Fuchs, bought the 38-story building in 2000 for $379 million. To entice employees back to the office, RFR last year built a $25 million “playground” in an underground garage that’s equipped with a climbing wall and pickleball and basketball courts. Four new tenants signed leases in the past few months, according to Trepp.

Even with all the vacant space, some landlords like Mr. Rechler’s RXR still want to build new towers. RXR is moving ahead with plans to build what could be one of the tallest buildings in the country at 175 Park Avenue.

“It’s one of a kind in what is and will always be one of the best office markets in the world,” he said, referring to the tower.