Kiev, Ukraine (CNN) - Ukraine's new leaders accused neighbor Russia of declaring war, as Kiev mobilized troops and called up military reservists in a rapidly escalating crisis that has raised fears of a conflict.
id 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 said Ukrainian troops continue to protect and serve Ukraine.
"This is a red alert. This is not a threat. This is actually a declaration of war to my country," Ukrainian interim Prime Minister Arseniy Yatsenyuk said.
Speaking in a televised address from the parliament building in the capital, Kiev, he called on Russian President Vladimir Putin to "pull back his military and stick to the international obligations."
"We are on the brink of the disaster."
A sense of escalating crisis in Crimea -- an autonomous region of eastern Ukraine with strong loyalty to neighboring Russia -- swirled with U.S. Secretary of State John Kerry condemning what he called Russia's "incredible act of aggression."
Speaking on the CBS program "Face The Nation," Kerry said several foreign powers are looking at economic consequences if Russia does not withdraw its forces.
"All of them, every single one of them are prepared to go to the hilt in order to isolate Russia with respect to this invasion," he said. "They're prepared to put sanctions in place, they're prepared to isolate Russia economically."
But Ukraine's ambassador to the United Nations said his country needs more than diplomatic assistance.
"We are to demonstrate that we have our own capacity to protect ourselves ... and we are preparing to defend ourselves," Yuriy Sergeyev said on CNN's "State of the Union." "And nationally, if aggravation is going in that way, when the Russian troops ... are enlarging their quantity with every coming hour ... we will ask for military support and other kinds of support."
In Brussels, Belgium, NATO ambassadors held 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. "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."
Lean to the West, or to Russia?
Ukraine, a nation of 45 million people sandwiched between Europe and Russia's southwestern border, has been plunged into chaos since the ouster a week ago of of President Viktor Yanukovych following bloody street protests that left dozens dead and hundreds wounded.
Anti-government protests started in late November when Yanukovych spurned a deal with the EU, favoring closer ties with Moscow instead.
Ukraine has faced a deepening split, with those in the west generally supporting the interim government and its European Union tilt, while many in the east prefer a Ukraine where Russia casts a long shadow.
Nowhere is that feeling more intense than in Crimea, the last big bastion of opposition to the new political leadership. Ukraine suspects Russia of fomenting tension in the autonomous region that might escalate into a bid for separation by its Russian majority.
Ukrainian leaders and commentators have compared events in Crimea to what happened in Georgia in 2008. Then, cross-border tensions with Russia exploded into a five-day conflict that saw Russian tanks and troops pour into the breakaway territories of South Ossetia and Abkhazia, as well as Georgian cities. Russia and Georgia each blamed the other for starting the conflict.
The crisis in Ukraine appeared to hit a boiling point after Putin obtained permission Saturday from his parliament to use military force to protect Russian citizens in Ukraine, spurning Western pleas not to intervene.
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.
The vote came as the newly installed pro-Russian leader of Crimea, Sergey Aksyonov, asked Putin for help in maintaining peace on the Black Sea peninsula, where Russia's fleet is based at Sevastopol. Security forces "are unable to efficiently control the situation in the republic," he said in comments broadcast on Russian state channel Russia 24. Aksyonov was installed as the region's premier after armed men took over the Crimean parliament building on Thursday.
Aksyonov said that a referendum on greater Crimean autonomy, originally set for May 25, would be moved to March 30.
At a Ukrainian parliamentary meeting Sunday, acting Defense Minister Ihor Tenyuh said Ukraine does not have the military force to resist Russia, according to two parliamentary members present at the session. Tenyuh called for talks to resolve the crisis with Russia, they said.
The Ukrainian National Security Council has ordered the mobilization of troops and the Defense Ministry was calling for reservists to register to be on standby if needed, a senior Ukrainian official, Andriy Parubiy, said.
Men dressed in both civilian and camouflage gear and wearing red armbands could be seen on the streets of the regional capital, Simferopol.
At Ukraine's Perevalnoye base, some 20 kilometers (12 miles) from Simferopol, a CNN team saw more than 100 troops -- not Ukrainian and dressed in black with no identifiable insignia -- deployed around its perimeter, as well as a dozen or so vehicles. Some 15 Ukrainian soldiers were on guard while civilians, both pro-Russia and pro-Ukraine protesters, stood on each side of the road.
In Kiev, thousands of people rallied in the central Independence Square, cradle of Ukraine's three-month anti-government protests that led to Yanukovych's ouster last week. The crowd held up signs reading "Crimea, we are with you" and "Putin, hands off Ukraine."
In Moscow, about 50 protesters were detained outside a Defense Ministry building, a Moscow police spokesman said.
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.
Western governments worried
The crisis set off alarm bells in the West.
In discussions over the weekend with Putin, U.S. President Barack Obama "made clear that Russia's continued violation of Ukraine's sovereignty and territorial integrity would negatively impact Russia's standing in the international community," according to a statement released by the White House.
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.
Canada recalled its ambassador to Moscow, while the United States and Britain announced they will suspend participation in preparatory meetings this week ahead of the G8 summit that will bring world leaders together in June in Sochi, Russia. France said it made the same decision.
Addressing the crowds in St. Peter Square, Pope Francis called for prayers for Ukraine, which he said was "living a delicate situation."
Pressure has been mounting on Russia as leaders from the EU and the UK joined an international outcry, with EU High Representative Catherine Ashton deploring Russia's "unwarranted escalation of tensions."
CNN's Victoria Eastwood and Diana Magnay reported from Simferopol, Ukraine; Ian Lee, Ingrid Formanek and Victoria Butenko from Kiev, while Marie-Louise Gumuchian and Steve Almasy wrote from London. CNN's Chelsea J. Carter, Bharati Naik, Richard Roth, Laura Smith-Spark, Tom Watkins, Sara Mazloumsaki, Alla Eshchenko, Arkady Irshenko, Radina Gigova and journalist Azad Safarov contributed to this report.