U.S. Secretary of State John Kerry told Russian Foreign Minister Sergei Lavrov in a telephone call on Saturday that any Russian steps to annex Ukraine's Crimea region would close the door to diplomacy, a U.S. State Department official said.
"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.
For his part, President Barack Obama, speaking to Baltic leaders amid rising tensions in Ukraine, told them that the United States supports them and is committed to its NATO defense commitments, the White House said.
"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 about the telephone call Obama held.
Obama spoke with Lithuanian President Dalia Grybauskaite, Latvian President Andris Berzins, and Estonian President Toomas Hendrik Ilves.
President Barack Obama also made a series of phone calls to world leaders about the ongoing crisis in Ukraine, including British Prime Minister David Cameron, French President Francois Hollande, and Italian Prime Minister Matteo Renzi, the White House said.
This comes as a Ukrainian border patrol plane came under fire while flying at about 1,000 metres (3000 feet) near the administrative border with Russian-occupied Crimea on Saturday, the border guards said.
No one was hurt when gunmen opened fire on the unarmed aircraft, a spokesman said. The Diamond light aircraft was flying three crew on an observation mission, the spokesman said.
Russian forces took control of Crimea last week. Pro-Moscow separatists have declared the region part of Russia and plan a referendum for March 16 to confirm it.
Moscow says it has no troops in Crimea beyond those normally stationed there with its Black Sea Fleet, an assertion ridiculed by the West.
Earlier in the day, Ukraine's acting foreign minister said his country would not give up Crimea and would do all in its power to resolve the crisis over the Black Sea peninsula peacefully.
"Crimea is and will be Ukrainian territory and we will not give up Crimea to anyone," Andriy Deshchytsia told a news conference in the Ukrainian capital Kiev.
Referring to deaths this year during protests against Ukraine's now deposed leader, Viktor Yanukovich, he said: "We are putting all our efforts into solving this matter through diplomacy - we have already had too many victims."
On Friday, pro-Russian activists continued to gather in front of the local administration building in the mining town of Donetsk, in eastern Ukraine.
Protesters demanded a referendum for greater autonomy in the region and asked for Russia's help.
Foreign military mission barred from Crimea:
This as an Associated Press reporter said that pro-Russian forces refused to let a foreign military mission enter Crimea on Saturday.
After the officers had stopped, the armed men fired warning bursts of automatic weapons fire into the air to make other unidentified vehicles halt. No injuries were reported.
The multinational group of military officers from the Organization for Security and Cooperation in Europe was attempting to enter the embattled peninsula from the north. The armed men told them they had no authorization to enter Crimea.
The OSCE mission will likely return to the Ukrainian city of Kherson where it had spent the night, the AP reporter said.
Russia and Ukraine are locked in a tense standoff over Crimea.
REUTERS/AP