TOKYO — The commander of U.S. troops in South Korea said he was heartbroken on Wednesday after taking the unprecedented step of putting about 4,000 local workers on unpaid leave — in the midst of the coronavirus epidemic — as the allies struggle to reach agreement on sharing defense costs.

The impasse stems from President Trump’s demand that Seoul raise more than fivefold its contribution to the cost of stationing about 28,500 U.S. troops in the country, a demand that South Korea says is politically impossible to meet.

Negotiators have since narrowed the gap between the two sides, and South Korea believes a deal may be close. But with money running out, the U.S. military command in South Korea placed nearly half the South Korean workers at its military bases on unpaid leave on Wednesday, for the first time in the history of the 70-year alliance.

“This is an unfortunate day for us. It’s unthinkable. It’s heartbreaking,” Gen. Robert B. Abrams, commander of U.S. Forces Korea, told the furloughed workers. The partial furlough “is not what we envisioned or hoped would happen,” he said, according to a transcript of his remarks.

A union representing the South Korean workers said that about 4,000 of the U.S. bases’ 8,500 local employees were furloughed.

On Tuesday, South Korean negotiator Jeong Eun-bo said the decision to furlough the workers “does not properly reflect the situation of the negotiations” and called it regrettable.

Jeong said that negotiations for a new deal were in their “final phase” and that he expected an agreement to be concluded soon.

“There seems to have been some progress, but the negotiations are still underway,” an official with South Korea’s presidential Blue House told reporters late Wednesday.

Abrams said the furlough was necessary because “programmed funds” had run out in the absence of a cost-sharing agreement.

Under the previous deal, South Korea paid about $870 million toward the cost of the U.S. troop presence, but Trump has said he wants a contribution of about $5 billion.

Abrams urged the governments in Seoul and Washington to reach a deal soon.

“These are our employees, our co-workers, our teammates, and we consider them family,” he said. “It is an understatement to say these are challenging times, as no one feels the impact more than our furloughed employees.”

Jonathan Pollack, a senior nonresident fellow at the Brookings Institution, pointed out the irony of the White House requesting assistance from South Korea to combat the novel coronavirus even as the Pentagon continues to push for more money from the country.

“It’s both a colossal mess and potentially a very dangerous one,” he said.

South Korea’s left-leaning Hankyoreh newspaper called the threat to the livelihoods of South Korean workers “a shame and a disgrace.”

“This unpaid leave, triggered by Trump’s unreasonable demands, reminds us once again of how far we are from a fair alliance,” it wrote in an editorial.