Russia-Ukraine ceasefire falls apart; B-52s spotted near Ukraine as Putin warns against no-fly zone

The overnight ceasefire between Russian and Ukrainian forces appeared to end Saturday morning, with fighting resuming as B-52 planes were spotted near Russia's border and Russian President Vladimir Putin warned allied forces against establishing a no-fly zone over Ukrainian airspace. 

The Ukrainian deputy prime minister said that Russia had violated the ceasefire agreement and that Russian forces were carrying out attacks in the cities of Volnovakha and Mariupol. 

The resumption of fire comes as Putin warned that "any move in [the] direction" of a no-fly space over Ukraine would be viewed as "a threat to our service members."

Ukraine President Volodymyr Zelenskyy and others have called for implementation of a no-fly zone, which NATO earlier this week rejected, to stop Russia air attacks.

However, critics say such a move would escalate tensions with Russia and bring additional forces directly into the conflict, potentially sparking a full-scale global war. 

Putin indicated as much Saturday when he strongly cautioned against authorities against imposing a no-fly zone.

"That very second, we will view them as participants of the military conflict, and it would not matter what members they are,” he said. 

U.S. B-52 bombers, meanwhile, were late this week spotted flying over Romania, a member of NATO that directly borders Ukraine. 

The planes were part of an exercise with the German and Romanian militaries.