(Updates with Kerry-Lavrov discussion; adds additional detail about calls)
By Arshad Mohammed and Roberta Rampton
WASHINGTON/KEY LARGO, Florida, March 8 (Reuters) - U.S. Secretary of State John Kerry warned Russia on Saturday that any steps to annex Ukraine's Crimea region would close the door to diplomacy, a U.S. State Department official said.
Kerry's latest telephone call with his Russian counterpart, Foreign Minister Sergei Lavrov, came as the standoff between occupying Russian forces and besieged Ukrainian troops intensified in Crimea.
"He made clear that continued military escalation and provocation in Crimea or elsewhere in Ukraine, along with steps to annex Crimea to Russia would close any available space for diplomacy, and he urged utmost restraint," the official said.
President Barack Obama sought to reassure nervous Baltic leaders on Saturday about U.S. support for their security and consulted other European allies about the Ukraine crisis.
Obama, who is on a weekend vacation with his family at a lush Florida resort, placed a series of calls to six world leaders who agreed Russia needs to pull its military forces back to their bases and allow international observers into Crimea, the White House said.
He convened a conference call with Lithuanian President Dalia Grybauskaite, Latvian President Andris Berzins, and Estonian President Toomas Hendrik Ilves. It was the first time he has spoken with the leaders of the three Baltic states about the crisis.
The countries are NATO members with strong economic ties to Russia, and have expressed nervousness about President Vladimir Putin's actions in Ukraine.
"The president reaffirmed the United States' unwavering commitment to our collective defense commitments under the North Atlantic Treaty and our enduring support for the security and democracy of our Baltic allies," the White House said in a statement.
The United States has moved to reassure its NATO allies, sending six more F-15 fighter jets this week to NATO's policing mission over the Baltic states. The jets are on call to respond to any violations of Baltic airspace.
Putin's justification for intervening in Ukraine to protect Russian speakers in Crimea has alarmed many in the Baltics, which used to be part of the Soviet bloc and have their own ethnic Russian minorities whose rights Moscow says are being undermined.
Obama made his third call this week about the crisis to British Prime Minister David Cameron and his second call in a week to French President Francois Hollande, and the leaders agreed on the diplomatic steps needed to defuse the tension in the region.
"They underlined that, if there was no progress was made in that direction, new measures would be taken that would sensibly affect relations between Russia and the international community, which is in no one's interest," Hollande's office said.
Obama also called Italian Prime Minister Matteo Renzi, his first call about Ukraine to the NATO member, the White House said. On Friday night, Obama telephoned German Chancellor Angela Merkel to discuss the situation in Ukraine. (Additional reporting by Astrid Wendladt in Paris; Editing by Sandra Maler)