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WASHINGTON - U.S. defense officials are telling Russia to tone down its tough talk on Ukraine, warning Moscow could inflame an already tense and fragile situation.

“We are extremely concerned,” Pentagon Deputy Press Secretary Gordon Trowbridge said Friday, emphasizing that Russia should do more to prevent further escalation.

“Russia is continuing this pattern of provocative rhetoric which has obviously created an impression in a lot of minds that there is something significant going on there,” he added.

Trowbridge also said the U.S. has not seen any evidence to support Russian claims that Ukraine has been trying to carry out terror attacks, citing what he called Moscow’s record of “frequently levying false accusations at Ukraine."

Earlier Friday in Moscow, Prime Minister Dmitry Medvedev said Russia could break off diplomatic ties with Ukraine in light of the recent security concerns.
 
"If there is no other way to change the situation, the president [Vladimir Putin] could take this step," state media quoted Medvedev as saying.

Crimea, Ukraine
Crimea

Diplomatic relations between Moscow and Kyiv remained intact after Russia annexed Crimea, former Ukrainian territory whose residents voted to secede, in 2014, or when Russia subsequently gave extensive material support to separatist rebels in eastern Ukraine who have declared themselves independent from Kyiv.
 
Russia has deployed an advanced air-defense missile system in Crimea, the military said in a statement Friday. The French news agency reports the anti-aircraft S-400 system is capable of tracking up to 300 targets and shooting down nearly three dozen simultaneously.

U.S. defense officials said they are monitoring the situation.

"We don't necessarily see any evidence of troop movements that are so large that we're concerned about those on their own," Trowbridge said.

Ukrainian troops were placed on high alert this week as tensions with Russia increased, both along the Crimean border and in eastern Ukraine.

Esha Sarai, Carolyn Presutti contributed to this report.