WASHINGTON, Jan 5 (AFP) - U.S. President Joe Biden suggested Vladimir Putin's struggle in Ukraine after 10 months of war and thousands of lives lost had prompted the Russian president to offer a 36-hour truce, saying, "I think he's trying to find some oxygen."

The Kremlin said Putin had ordered a ceasefire from midday on Friday after a call for a Christmas truce by Patriarch Kirill of Moscow, head of the Russian Orthodox Church.

Ukraine spurned the ceasefire offer over Russia's Orthodox Christmas, saying there would be no truce until Moscow withdraws its invading forces from occupied land.

Asked about the proposed truce, Biden told reporters at the White House: "I'm reluctant to respond to anything that Putin says. I found it interesting that he was willing to bomb hospitals and nurseries and churches ... on the 25th and New Year's. I mean, I think he's trying to find some oxygen."

Russia's ambassador in Washington, Anatoly Antonov, accused the U.S. administration of lacking any desire for a political settlement, adding that "even" the unilaterally declared ceasefire was being labelled an attempt to find some oxygen.

"All this means that Washington is set on fighting with us 'to the last Ukrainian,' and the fate of Ukraine's people does not worry the Americans at all," Antonov said in remarks on the embassy's Facebook page that were framed as responses to media questions.

Putin's ceasefire would begin in time for observation of Christmas by Russia's Orthodox Church on Jan. 7. Ukraine's main Orthodox Church has been recognized as independent by the church hierarchy since 2019 and rejects any notion of allegiance to the Moscow patriarch. Many Ukrainian believers have shifted their calendar to celebrate Christmas on Dec. 25 as in the West.

It would be the first major truce of the more than 10-month-old war that has claimed tens of thousands of lives and devastated large swaths of Ukraine.