BRUSSELS (AP) -- NATO foreign ministers moved Tuesday to beef up the defenses of front-line alliance members feeling menaced by a more assertive Russia, with Secretary of State John Kerry proclaiming the U.S. commitment to their security is "unwavering."
The ministers from NATO's 28 member nations also ordered suspension of all "practical civilian and military cooperation" with Vladimir Putin's Russia, though they made sure a line of communication with the Kremlin remains open at the ambassadorial level.
The North Atlantic Treaty Organization, keystone of U.S. and European security since the end of World War II, is facing its most acute geopolitical crisis in years: the fallout from Moscow's unilateral annexation of Ukraine's Crimean Peninsula, which the Obama administration and its allies condemn as a brazen, illegal land grab.
On Tuesday, an estimated 35,000 to 40,000 Russian troops equipped with tanks, other armored vehicles and fixed and rotary wing aircraft remained positioned near the border with Ukraine, a NATO military official told The Associated Press, speaking on condition of anonymity because of the sensitive nature of the information.
The military official described the Russian buildup as "a complete combat force" that was highly threatening to Ukraine.
Those troops, and future aggressive moves that Putin's Kremlin may make, have become a troubling concern for NATO countries, especially the alliance's eastern-most members -- the Baltic states, Poland, and Romania, all of which were once in Moscow's orbit.
To reassure those skittish allies, Kerry told a news conference, the U.S. has already sent six F-15 fighters to perform air patrols over the Baltic and deployed a dozen F-16s to Poland. He noted that the U.S. had previously dispatched the USS Truxtun, a guided-missile destroyer, to the Black Sea.
The Truxtun has already left the Black Sea after taking part in naval exercises with NATO allies Romania and Bulgaria that had been scheduled before the Ukraine crisis.
A senior U.S. defense official said the U.S. is likely to send another warship to the Black Sea, but there was no decision on which ship and when it would be sent. The official, who was not authorized to speak publicly about the matter, spoke on condition of anonymity.
"And more U.S. support is on the way," Kerry said.
At the Tuesday afternoon meeting Kerry attended with the NATO foreign ministers, the secretary said, "more allies pledged their own contributions to make sure every ally from the Baltic to the Black Sea feels secure." He did not provide details.
The defense official said the U.S. was also likely to send a small team of soldiers to Europe and was encouraging NATO allies to contribute more aircraft to the Baltic air patrol mission.
Despite annexing Crimea, Putin and other Kremlin officials have said that Russia has no intention of invading other areas of Ukraine. Defense Minister Sergei Shoigu insisted Tuesday the Kremlin wants a "political settlement that would take interests and rights of the entire Ukrainian people into account."
Meeting behind closed doors, Kerry and his Canadian and European colleagues agreed unanimously on steps NATO must take in response to Russian actions.
A civilian alliance official who attended the session and briefed reporters afterward on condition of anonymity said the measures include possible deployment and reinforcement of military assets in NATO member countries that feel Moscow's actions may pose a security threat, as well as an increase of readiness levels for NATO rapid response forces. Other steps include a review of NATO's crisis response plans, as well as military training and exercise schedules.
While NATO ordered the suspension of "all practical civilian and military cooperation" with Russia, Secretary General Anders Fogh Rasmussen said later he thought some mutually beneficial cooperation programs with the Russians might continue, such as the project to train anti-narcotics personnel in Afghanistan, Pakistan and Central Asia.
The four-star U.S. Air Force general who is NATO's supreme military commander, Philip Breedlove, has been ordered to devise plans to provide visible reassurance to alliance countries in Central and Eastern Europe that other NATO nations are ready to help protect them "on land, air and at sea," Kerry said.
America's commitment to NATO's core principle -- that an attack on one alliance member shall be considered an attack on all -- is "unwavering," the secretary said.
NATO ministers also met Tuesday with Ukrainian Foreign Minister Andriy Deshchytisa. Kerry said "they stood together in defense of Ukraine's right to choose its future and in defense of international law." Kerry blasted what he called Russia's "tactics of intimidation, especially deployment of unprecedented forces around Ukraine's borders."