By signaling its openness to a ceasefire, Ukraine has handed the Kremlin a difficult challenge at a time when the Russian military has the upper hand in the war: Should Moscow accept a truce and abandon hopes of making new gains, or should it reject the offer and risk derailing a cautious rapprochement with Washington?

Russian President Vladimir Putin has repeatedly ruled out a temporary break in hostilities, saying it would only benefit Ukraine and its Western allies by letting them replenish their arsenals. He has insisted Moscow wants a comprehensive agreement that would ensure a lasting settlement.

The Kremlin responded cautiously to the news of Ukraine accepting the U.S.-proposed truce during Tuesday’s talks in Saudi Arabia, saying that it needs to know details of the discussions before expressing its view.

The careful approach reflects Putin’s awareness of the risk that a blunt rejection of the offer could upset tentative efforts to normalize Russia-U.S. ties.

Observers say that instead of an outright rejection, Putin will likely propose linking the truce to certain conditions that would protect Moscow’s interests.

The Russian military held the battlefield initiative last year, making slow but steady gains along several sections of the 600-mile front line. The tempo of Russian advances accelerated in the fall, when Moscow’s forces captured the most territory since the start of the war.