With President Trump and President Vladimir V. Putin preparing to meet in Alaska on Friday, ending the Russian leader’s years of diplomatic isolation with the West, Ukraine’s sidelined leaders warned that the Kremlin would try to “deceive America” as Mr. Trump pushed to end the war.

Ukraine’s president, Volodymyr Zelensky, is determined to demonstrate that his country is not the obstacle to peace by emphasizing Kyiv’s willingness to accept an unconditional cease-fire, an idea that Russia has rejected. He has cautioned that Mr. Putin will try to drive a wedge between the United States, Ukraine and its European allies by putting forth demands that the Kremlin knows Ukraine cannot accept and then portraying Mr. Zelensky as the barrier to a deal.

“We understand the Russians’ intention to try to deceive America — we will not allow this,” Mr. Zelensky said in his evening address to the nation on Sunday night.

Ukraine and its European allies are pressing hard for Mr. Zelensky to be included in the negotiations, fearing the consequences of bilateral talks that exclude them, given maximalist demands by Mr. Putin that Ukraine says threatens its survival.

The stakes of the diplomacy are high. The bloodiest war on the continent since World War II has become deadlier and the warring sides have expanded arsenals capable of striking deep into enemy territory.