A group of 1,000 armed men were blocking the entrance to a unit of Ukraine's border guards in a tense standoff in the south of the flashpoint Crimea peninsula on Sunday, the defence ministry said.
"One thousand armed fighters and around 20 trucks are blocking the perimetre of the 36th brigade of border guards... in Perevalne," the ministry said in a statement, as tensions remain high after Russia's parliament approved the deployment of troops in Ukraine. It did not indicate what nationality the armed men were.
The outnumbered Ukrainians placed a tank at the base's gate.
In Kiev, Ukraine's new prime minister urged Russian President Vladimir Putin to pull back his military, warning that "we are on the brink of disaster."
Prime minister Arseniy Yatsenyuk spoke a day after Russian forces took over the strategic Black Sea peninsula of Crimea from Ukraine without firing a shot.
"There was no reason for the Russian Federation to invade Ukraine," Yatsenyuk said after a closed session of his new parliament in Kiev.
So far, the new government in Kiev has been powerless to react to Russian military tactics. Armed men in uniforms without insignia have moved freely about the key peninsula, occupying airports, smashing equipment at an air base and besieging a Ukrainian infantry base.
Russia has its key Black Sea Fleet stationed on the Crimean peninsula — which was formerly part of Russia until 1954 — and nearly 60% of Crimea's residents identify themselves as Russian.
Putin has defied calls from the West to pull back his troops, insisting that Russia has a right to protect its interests and Russian-speakers in Crimea and elsewhere in Ukraine. However, there has been no sign of ethnic Russians facing attacks in Crimea or elsewhere in Ukraine.
President Barack Obama spoke with Putin by telephone for 90 minutes on Saturday and expressed his "deep concern" about "Russia's clear violation of Ukrainian sovereignty and territorial integrity," the White House said. Obama warned that Russia's "continued violation of international law will lead to greater political and economic isolation."
Unidentified troops pulled up to the Ukrainian military base at Perevalne on the Crimea peninsula Sunday in a convoy that included at least 13 trucks and four armored vehicles with mounted machine guns. The trucks carried 30 soldiers each and had Russian license plates.
A dozen Ukrainian soldiers, some with clips in their rifles, stood on the other side of the military gate. Neither side would speak to AP journalists.
The new Ukrainian government came to power last week following months of pro-democracy protests against a pro-Russian president, Viktor Yanukovych, and his decision to turn Ukraine toward Russia instead of the European Union. Yanukovych fled to Russia after more than 80 people died, most of them demonstrators killed by police. He insists he's still president.
Since then, tensions have risen sharply between the two capitals.
Ukraine's acting president, Oleksandr Turchynov, announced late Saturday that he had ordered Ukraine's armed forces to be at full readiness because of the threat of "potential aggression." He also said he had ordered stepped-up security at nuclear power plants, airports and other strategic infrastructure.
Ukraine's population of 46 million has divided loyalties between Russia and Europe, with much of western Ukraine advocating closer ties with the EU, while eastern and southern regions look to Russia for support.
The Interfax news agency reported the speaker of Crimea's legislature, Vladimir Konstantinov, as saying the local authorities did not recognize the government in Kiev. He said a planned referendum on March 30 would ask voters about the region's future status.
But the US and other Western governments have few options to counter Russia's military moves.
Nato's secretary general said Russia had violated the UN charter with its military action in Ukraine, and he urged Moscow to "'de-escalate the tensions." Nato chief Anders Fogh Rasmussen spoke in Brussels as he was about to open a meeting Sunday of the alliance's political decision-making body to discuss the crisis.
Ukraine is not a Nato member, meaning the US and Europe are not obligated to come to its defence. But Ukraine has taken part in some alliance military exercises and contributed troops to its response force.
The US also said it will suspend participation in preparatory meetings for the Group of Eight economic summit planned in June to be held at Russia's Black Sea resort of Sochi, site of the 2014 Winter Olympics.
French foreign minister Laurent Fabius agreed, saying on Europe 1 radio that planning for the summit should be put on hold.
France "condemns the Russian military escalation" in Ukraine, and Moscow must "realize that decisions have costs," he said Sunday.