President Donald Trump on Monday sought to increase pressure on Russian President Vladimir Putin in a bid to secure an end to Moscow's full-scale invasion of Ukraine.

Earlier in July, Trump set a 50-day deadline for the Kremlin to reach a truce with Kyiv or risk economic penalties, but the warning has not halted Russia's barrage of strikes. The wave of attacks came as Russia said its troops were pushing deeper into Ukrainian territory.

Russia and Ukraine discussed further prisoner swaps on Wednesday at a brief session of peace talks in Istanbul, but the sides remained far apart on ceasefire terms and a possible meeting of their leaders.