Ukrainian President Volodymyr Zelensky said on Wednesday that he expected strong measures from the United States if Russia rejected Washington's proposal for a 30-day ceasefire with Ukraine.

"I understand that we can count on strong steps. I don't know the details yet but we are talking about sanctions (against Russia) and strengthening Ukraine," Zelensky said.

Zelensky also said Kyiv had no "trust" in Russia, despite having agreed to put a US proposal for a 30-day truce to Moscow. "I have emphasised this many times, none of us trust the Russians," Zelensky said at a press conference in Kyiv.

Meanwhile, German Chancellor Olaf Scholz said Wednesday that any ceasefire in Ukraine must be part of a process that leads to a "lasting agreement", after Kyiv backed a US-proposed 30-day truce with Russia.

"Some things are very important and central -- a ceasefire must be embedded in a peace process that leads to a lasting agreement," Scholz said at a press conference in Berlin alongside EU Council chief Antonio Costa.

"Ukraine's sovereignty must be preserved, and Ukraine needs strong armed forces as the most important security guarantee for the country," he added.

"The demilitarisation of Ukraine has been one of Russia's war goals, and that must not succeed."