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Secretary of State John Kerry on Friday warned Russia against sparking further unrest in Ukraine following reports that armed Russian militants were patrolling an airport there.

Kerry said he spoke with his Russian counterpart, Foreign Minister Sergei Lavrov, and told him that Russian intervention in the already volatile Ukraine would be a "grave mistake."

Kerry said he raised concerns over reports that Russian armored trucks and troops were seen in the streets of the Crimean city of Sevastopol and other reports that armed soldiers flooded two Ukrainian airports.

"The question is whether or not what is happening now might be crossing a line in any way and we are going to be very careful in making our judgments about that," Kerry said.

Kerry warned that Russia must respect Ukraine's territorial integrity, and Lavrov assured Kerry that Putin does not intend to violate the country's sovereignty.

"I nevertheless made it clear that that could be misinterpreted at this moment and that there are enough tensions that it is important for everybody to be extremely careful not to inflame the situation and not to send the wrong messages," Kerry said.

Kerry stressed that the goal was to stabilize the situation and the economy in Ukraine, and said while Russia has a base in Ukraine, they must be careful not to violate the rules that apply to their presence.

— Catherine Chomiak