President Trump said the United States would impose sanctions on Russia if it did not accept an extended cease-fire in its war with Ukraine, writing on social media after a phone call late Thursday with President Volodymyr Zelensky of Ukraine.

The remark was another swivel by the administration closer to Ukraine’s positions in cease-fire talks, after two months of concessions to Russia including broadening diplomatic relations and voting against a U.N. resolution condemning Moscow for the war.

During talks in Saudi Arabia in March, the Trump administration had proposed a 30-day truce to allow for peace negotiations. Ukraine accepted the idea but Russia has pushed instead for negotiating terms for a settlement before accepting a cease-fire.

After a meeting in Vatican City with Mr. Zelensky on the sidelines of Pope Francis’s funeral last month. Mr. Trump threatened Russia with sanctions if it drew out of the negotiations. But his social media post after his call with Mr. Zelensky was his clearest statement yet that he was linking additional U.S. sanctions on Russia to its acceptance of a 30-day truce for negotiations.

“If the ceasefire is not respected, the U.S. and its partners will impose further sanctions,” Mr. Trump wrote in the post, made on Truth Social, his social networking site.

The United States, Mr. Trump wrote, “calls for, ideally, a 30-day unconditional ceasefire.”.

Mr. Zelensky’s office published a readout of the call with Mr. Trump saying the two leaders had congratulated one another on the anniversary of the end of World War II in Europe. It also said that on the call, “President Trump confirmed that he wants to see this war ended and is ready to help.”