Ukraine's new Prime Minister Arseniy Yatsenyuk warned Sunday his crisis-hit country was on the "brink of disaster," accusing neighbor Russia of declaring war.

Ukraine's shaky new government mobilized troops and called up military reservists Sunday, even as the defense minister said Kiev stood no chance against Russian troops in a rapidly escalating crisis that has raised fears of a conflict.

Amid signs of Russian military intervention in Ukraine's Crimean peninsula, Russian generals led their troops to three bases in the region Sunday demanding Ukrainian forces surrender and hand over their weapons, Vladislav Seleznyov, spokesman for the Crimean Media Center of the Ukrainian Defense Ministry, told CNN.

Speaking by phone, he said Russian troops had blocked access to the bases, but added "there is no open confrontation between Russian and Ukrainian military forces in Crimea" and that Ukrainian troops continue to protect and serve Ukraine.

"This is a red alert. This is actually a declaration of war in our country," Yatsenyuk said.

Speaking in a televised address from the parliament building in Kiev, Yatsenyuk called on Russian President Vladimir Putin to "pull back the military and stick to international obligations."

"We are on the brink of disaster," he said.

In Brussels, Belgium, NATO ambassadors were scheduled to hold an emergency meeting on Ukraine.

"What Russia is doing now in Ukraine violates the principles of the U.N. charter," NATO Secretary General Anders Fogh Rasmussen told reporters.

"Russia must stop its military activities and threats," Rasmussen said, adding, "we support Ukraine's territorial integrity and sovereignty. ... We support the rights of the people of Ukraine to determine their own future without outside interference."

Escalating crisis

Ukraine's parliament met behind closed doors Sunday. At the closing of the session, acting Defense Minister Ihor Tenyuh said Ukraine does not have the military force to resist Russia, according to two parliamentary members present at the meeting. Tenyuh called for talks to resolve the crisis with Russia, they said.

The Ukrainian National Security Council has ordered the mobilization of troops, as Putin appeared to dismiss warnings from world leaders to avoid military intervention in Crimea, a senior Ukrainian official, Andriy Parubiy, said.

He also said the Defense Ministry was calling for reservists to register at the local and regional headquarters to be on standby if needed.

A sense of escalating crisis in Crimea -- an autonomous region of eastern Ukraine with strong loyalty to neighboring Russia -- swirled Saturday night, with U.S. Secretary of State John Kerry condemning what he called "the Russian Federation's invasion and occupation of Ukrainian territory."

Russia has not confirmed it deployed thousands of troops to the region following reports that armed, Russian-speaking forces wearing military uniforms -- without insignia -- patrolled key infrastructure sites.

It was the latest in a series of fast-moving developments that saw Russia's parliament sign off on Putin's request to send military forces into Ukraine, raising the stakes in the escalating brinksmanship. Putin cited in his request a threat posed to the lives of Russian citizens and military personnel based in southern Crimea. Ukrainian officials have vehemently denied Putin's claim.

According to a tweet from the official Russian government account Sunday, Russian Prime Minister Dmitry Medvedev discussed the crisis in Ukraine in a telephone call with Yatsenyuk.

According to a second tweet, Medvedev said Russia is interested in maintaining stable and friendly relations with Ukraine but reserves the right to protect the legitimate interests of its citizens and military personnel stationed in Crimea.

Path to war?

In Kiev, thousands of people rallied in the central Independence Square, cradle of Ukraine's three-month anti-government protests that led to President Viktor Yanukovych's ouster last week.

A crowd held up signs reading "Crimea, we are with you" and "Putin, hands off Ukraine."

Putin's move prompted world diplomats to call for a de-escalation of tensions that have put the two neighbors on a possible path to war and roiled relations between Russia and the United States.

In what appeared to be an illustration of the growing schism between the two world powers, U.S. President Barack Obama and Putin spoke for 90 minutes, with each expressing his concern over the mounting crisis, according to separate statements released by their governments.

According to the Kremlin, Putin told Obama that Russia reserves the right to defend its interests in the Crimea region and the Russian-speaking people who live there.