Two U.S. senators are warning that Russian President Vladimir Putin is stalling on peace while preparing a fresh military offensive against Ukraine.

The drone attacks came on the eve of expected peace talks in Istanbul between Russia and Ukraine, which were already strained by uncertainty and pressure from U.S. President Donald Trump.

Russia has proposed another attempt at peace negotiations with Ukraine on Monday in Istanbul, but both sides remain firmly entrenched in positions likely to prolong the war.

Russian President Vladimir Putin’s conditions for ending the war in Ukraine include a demand that Western leaders pledge in writing to stop enlarging NATO eastwards, according to reports.

Trump previously said his personal relationship with Putin would usher in peace for Ukraine. Now, he says Putin's gone "absolutely CRAZY!"

The U.S. president lashed out at the Kremlin on Tuesday as the White House continued to consider increasing sanctions against Russia.

Hundreds of towns are under evacuation orders in Ukraine’s northeastern Sumy region where Russian forces are focusing their attacks, as Kyiv officials warn it could be preparing a new offensive.Mandatory evacuation orders were issued for 11 settlements on Saturday,

Retired U.S. military leaders reacted on Sunday after Ukraine carried out a large-scale drone attack on Russian military aircraft deep inside the country, with one quipping that Russian President Vladimir Putin is "having a very bad day.