The U.S. military is prepared to back up NATO if the unrest in Ukraine escalates, Joint Chiefs Chairman Gen. Martin Dempsey said Friday.

American diplomats are working around the clock to come to a solution to Russia’s takeover of Crimea in eastern Ukraine. Dempsey said he’s been talking to his military counterparts in Russia, but he’s also sending a clear message to Ukraine and members of NATO that the U.S. military will respond militarily if necessary.

“We're trying to tell [Russia] not to escalate this thing further into Eastern Ukraine, and allow the conditions to be set for some kind of resolution in the Crimea,” Dempsey said in an interview with PBS Newshour’s Judy Woodruff.

“But the message we are sending militarily is to our NATO allies,” he said.

When asked if there is a chance of U.S. military intervention in the Ukraine crisis, Dempsey replied, “That’s a question that I think deserves to be assessed and reassessed and refreshed as this thing evolves.”

“But remember,” he added, “we do have treaty obligations with our NATO allies. And I have assured them that if that treaty obligation is triggered, we would respond.”

Early Thursday the American guided-missile destroyer, USS Truxtun, departed Greece headed toward the Black Sea. The destroyer will be passing through the region as part of the USS George H.W. Bush Carrier Strike Group bound for the Middle East. This trip, Kirby explained, had been planned well before the February unrest in Kiev began and new leadership took control of the embattled nation.

Some estimates this week put the numbers of Russian troops in Crimea—which Russian President Vladimir Putin maintained Tuesday are merely “self-defense” forces—at 30,000. But Pentagon spokesman Rear Adm. John Kirby said Friday that despite the inherent difficulties of arriving at an exact figure of Russian troops, the latest intelligence suggests the number is closer to 20,000.

Russia has “added forces over the last week or so,” he said. “It's less important the number than what it is they’re doing...blocking off Ukrainian naval bases by sinking patrol boats into the waterways,” for example, he added, pointing to the latest developments around the two naval bases Russia leases from Ukraine.

Defense Secretary Chuck Hagel called his Ukrainian counterpart, Defense Minister Ihor Tenyukh, on Friday, speaking for the first time since Tenyukh assumed the job after pro-Russian President Viktor Yanukovych fled the country. The call lasted about 20 minutes and Tenyukh asked for advice on carrying out humanitarian missions, Kirby said.

Kirby emphasized the Pentagon’s ties to its allies in the region, explaining that the U.S. will continue to “look for ways to demonstrate our commitment to NATO” in the coming days and weeks.

“As Secretary Hagel said yesterday, we will pursue measures that reinforce those commitments to include the provision of additional support to NATO's Baltic air policing mission and to our aviation detachment in Poland,” said Kirby.

That Baltic policing mission currently includes 10 U.S. personnel working alongside Polish troops.

In addition to sending F-15s to the Baltic and F-16s to Poland, the Obama administration this week imposed several measures in response to Russian maneuvers in Ukraine.

“In terms of how that's manifested, you have seen broad international unity in the condemnations out of the North Atlantic Council, out of our European allies, and out of the G7 countries,” a senior administration official said. “We suspended preparatory meetings for the G8 in Sochi. The United States has pulled down and cancelled discussions associated with keeping trade and commercial ties to Russia. We've also cancelled military exercises and joint consultations with Russia on those specific issues, while providing additional reassurance to our European allies about our commitment to their security.”

Nevertheless, Dempsey cautioned, “If Russia is allowed to do this, which is to say move into a sovereign country under the guise of protecting ethnic Russians in Ukraine, it exposes Eastern Europe to some significant risk, because there are ethnic enclaves all over Eastern Europe and the Balkans.”

But, he added, “I do have this open line with my Russian counterparts. So everything that we’ve done, I tell them, here’s what we’re doing. Here’s why we’re doing it, you know, we disagree fundamentally about your claim of legitimacy; but as militaries, let’s try to avoid escalating this thing.”