Nurses at MedStar Washington Hospital Center went on what their union described as a one-day strike Monday, but the hospital said that the striking nurses could not return to work for 10 days.

The union National Nurses United, which represents the registered nurses at the hospital, has been engaged in negotiations with the hospital to replace a contract which expired in mid-November.

In a statement, the union said it was concerned about understaffing, unsafe conditions and inadequate pay for nurses. It notified the hospital 10 days in advance that the nurses would strike for one day.

Union marketing director Fernando Losado said that the “great majority” of the 1,857 nurses at the hospital refused to come in to work on Monday, and that about 500 at a time stood at the picket line during the day.

But Gregory Argyros, the hospital’s chief medical officer, said that 70 percent of the nurses scheduled to work Monday did show up.

Anyone who did not arrive for work will not be allowed to work again until Jan. 1, the hospital said, since it signed a 10-day contract to hire replacement nurses.

“The staffing agency requires that we give them 10 days to staff replacements. It is a contract requirement, especially with a hospital our size and the complexity of the patients. We have to bring in these nurses who are expert at the patients that we treat,” hospital spokeswoman So Young Pak said.

Argyros said, “The union and our nurses were notified before the contract ran out back in November of this fact. This is a commitment that we had to make.”

The hospital said in a statement that the nurses who cannot work until Jan. 1 are not eligible for holiday pay for either Christmas or New Year’s, since their contract requires them to work the day before a holiday in order to receive holiday pay.

Losado characterized the 10-day freeze for those nurses as a lock-out, and protested against it. “It is really clearly in our view a form of intimidation,” he said. “We think it’s nothing but an attempt to prevent nurses from exercising their rights.”

Argyros said that the hospital handled more than 150 surgery cases on Monday, making it a busier day than usual, and that he believed the quality of care was not lessened by the strike.