Earlier Friday, Defense Secretary Chuck Hagel phoned his Ukrainian counterpart, new Defense Minister Ihor Tenyuh, to show support for Ukrainian sovereignty, said Rear Adm. John Kirby, the Pentagon press secretary.
Tenyuh asked for U.S. technical advice on humanitarian relief and disaster operations but made no immediate requests for U.S. weaponry or other assistance, Kirby said at a Pentagon briefing.
The U.S. now estimates that Russia has about 20,000 troops in the Crimea in a "clear violation of lots of international obligations" on the part of Moscow, Kirby said.
"They have added forces over the last week or so," Kirby said, and the Russians were "essentially establishing operational control over the Crimea."
The U.S. Navy has ordered the guided missile destroyer Truxton into the Black Sea in what the Navy calls a routine visit unrelated to events in Ukraine.
The 510-foot Arleigh Burke class Truxton passed through the Bosphorus strait Friday into the Black Sea, where the Russian fleet reportedly has demanded the surrender of Ukrainian ships and sailors in Crimea.
The Truxton, part of the carrier George H.W. Bush battle group in the Mediterranean, was on a "regularly scheduled excursion" for training exercises in the Black Sea with the Bulgarian and Romanian navies, Kirby said.
"The naval forces of all three nations have stated that there is no connection between the upcoming drill and the tensions on Ukraine's Crimean peninsula, where the Russian Black Sea fleet is based," Bulgarian National Radio reported.
The passage of the Truxton to the Black Sea was part of a slow and measured buildup of U.S. forces on Russia's borders to assure European allies.
In Warsaw, Polish Defense Minister Tomasz Siemoniak welcomed the U.S. decision to send 12 F-16 fighters and 300 U.S. personnel to Poland next week for a training exercise called in response to the Ukraine crisis, according to Poland's Defense Ministry.
Siemoniak said that the exercise was originally planned to be smaller but Poland requested that it be strengthened after the Russian incursion in Crimea.
"When we face such a dramatic challenge for our security, we need to reassure our allies that our security guarantees are valid," said Stephen Mull, the U.S. ambassador to Poland.
Also on Friday, six F-15 fighters and a KC-130 tanker arrived in Lithuania to join air police patrols guarding the Baltic states.
The fighters and 60 U.S. military personnel landed at Siauliai Air Base in Lithuania, joining four F-15s and 150 troops already there for the air patrol mission.
Lithuanian President Dalia Grybauskaite said that "Lithuania highly appreciates the rapid NATO decision to step up forces in the region" in the face of Russian threats, the Lithuania Tribune reported.
"This proves that Lithuania and the entire Baltic region have real defense guarantees," Grybauskaite said.
On Thursday in Crimea, Russia's Black Sea Fleet scuttled two old ships in Sevastopol harbor channels to block any attempt by Ukrainian ships to leave.
Ukrainian Navy Capt. Viktor Shmiganovsky also said that Adm. Alexander Vitko, commander of the Russian fleet, came to a Ukrainian navy base earlier this week to urge the sailors to pledge allegiance to the Black Sea Fleet, promising good pay, the Los Angeles Times reported.
Vitko said the Ukrainian sailors should join the Russians "to help protect Ukraine from extremist gangs," Shmiganovsky said.
The Ukrainian navy base commander told Vitko: "Comrade Admiral, we didn't see any extremist gangs here until you came." Vitko left "in a rage," Shmiganovsky said.
-- Richard Sisk can be reached at email@example.com