The sudden arrival of men in military uniform patrolling key strategic facilities prompted Ukraine to accuse Russia of "military invasion and occupation" — a claim that brought an alarming new dimension to the crisis.
Associated Press journalists in Crimea spotted a convoy of nine Russian armored personnel carriers on a road between the port city of Sevastopol, where Russia has a naval base, and the regional capital, Simferopol. The tensions at two Crimea airports apparently caused the closure of airspace over the peninsula.
Russia kept silent on claims of military intervention, even as it maintained its hard-line stance on protecting ethnic Russians in Crimea, a territory that has played a symbolic role in its national identity.
Appearing for the first time since fleeing Ukraine, Yanukovych struck a tone both of bluster and caution — vowing to "keep fighting for the future of Ukraine," while ruling out seeking Russian military help.
"Any military action in this situation is unacceptable," he told reporters in the southern city of Rostov-on-Don near the border with Ukraine. In his closing remarks, seeking to make a firm point, Yanukovych tried — and failed — to break a pen.
Western powers, meanwhile, pressured Moscow to exercise restraint.
U.S. Secretary of State John Kerry said he had spoken with Russian Foreign Minister Sergey Lavrov — and delivered a warning against military moves that could further inflame tensions.
German Chancellor Angela Merkel spoke with Putin and expressed concern about an escalation in Crimea, her government said. The Kremlin said that Putin also had conversations with British Prime Minister David Cameron and EU President Herman Van Rompuy.
In Kiev, Ukraine's newly named interior minister accused Russia of military aggression.
"I can only describe this as a military invasion and occupation," Avakov wrote in a Facebook post.
The chief of Ukraine's security council, Andriy Parubiy, softened the tone later in the day, saying gunmen had tried to "seize" the airports in the Crimean cities of Simferopol and Sevastopol but insisting that "de facto the airports are controlled by the law enforcement bodies of Ukraine."
Parliament adopted a resolution demanding that Russia halt steps it says are aimed against Ukraine's sovereignty and territorial integrity, and called for a U.N. Security Council meeting on the crisis.
Ukraine's State Border Guard Service said about 30 Russian marines from Russia's Sevastopol-based Black Sea Fleet had taken up position outside the Ukrainian Coast Guard base in the area. It claimed the marines said they were there to prevent any weapons at the base from being seized by extremists.
Experts said Russia's hand in Crimea was clear — whether or not the armed men at the airports and the Coast Guard base were actually Russian soldiers.
"I think there two possibilities. One is that these are Russian forces, the second possibility us some kind of special paramilitaries," said Steven Pifer, a former U.S. ambassador to Ukraine who is now an analyst at the Brookings Institution.
The Russian armored personnel vehicles seen by AP reporters were parked on the side of the road near the town of Bakhchisarai, apparently because one of them had mechanical problems.
Russia is supposed to notify Ukraine of any troop movements outside the naval base it maintains in Sevastopol under a lease agreement with Ukraine.
The Russian Foreign Ministry said movements of armored vehicles belonging to the Russian Black Sea Fleet were prompted by the need to ensure security of its base and didn't contradict the lease terms.
A duty officer at the Ukrainian Defense Ministry said it had no information about the vehicles' movements.
AP journalists approaching the Sevastopol airport found the road leading up to it blocked by two military trucks and a handful of gunmen wearing camouflage uniforms and carrying assault rifles.
A car with Russian military plates was stopped at the roadblock. A man wearing a military uniform with a Russian flag on his sleeve got out of the car and was allowed to enter on foot after a brief discussion with the gunmen.
Ukraine International Airlines said it the closure of airspace had caused it to cancel flights to and from Simferopol airport on Friday evening and Saturday. The announcement did not say who had closed the airspace.
A woman answering Simferopol airport passenger help line said the airport was not closed but that all flights were canceled "due to the situation in Crimea."
At the airport, dozens of armed men in military uniforms without markings patrolled with assault rifles. They didn't stop or search people leaving or entering the airport, and refused to talk to journalists.
One man who identified himself only as Vladimir said the men were part of the Crimean People's Brigade, which he described as a self-defense unit ensuring that no "radicals and fascists" arrive from other parts of Ukraine. There was no way to verify his account.
The airport deployments came a day after masked gunmen with rocket-propelled grenades and sniper rifles seized the parliament and government offices in Simferopol and raised the Russian flag. Ukrainian police cordoned off the area but didn't confront the gunmen. They remained in control of the buildings Friday.
In Kiev, the prosecutor-general's office in Kiev said it would seek Yanukovych's extradition to Ukraine, where he is wanted on suspicion of mass murder in last week's violent clashes between protesters and police, during which over 80 people were killed.
Russia continued with massive combat readiness exercises involving most of its troops in western and southern Russia that it said were unrelated to the Ukraine conflict.
The Kremlin, in a statement published late Thursday, said Putin had instructed the government to "maintain contacts with the counterparts in Kiev in what concerns trade and economic ties between Russia and Ukraine."
Meanwhile, Swiss prosecutors announced they had launched a criminal investigation against Yanukovych and his son Aleksander over "aggravated money laundering." They said police and Geneva's chief prosecutor conducted a search and seized documents Thursday at the premises of a company owned by Aleksander Yanukovych.
Switzerland, Austria and Liechtenstein all said they would freeze any assets Yanukovych and his entourage might have in those countries.
Ukraine's population is divided in loyalties between Russia and the West, with much of western Ukraine advocating closer ties with the European Union while eastern and southern regions look to Russia for support.
Crimea, a southeastern peninsula of Ukraine that has semi-autonomous status, was seized by Russian forces in the 18th century under Catherine the Great, and was once the crown jewel in Russian and then Soviet empires.
It became part of Ukraine in 1954 when Soviet leader Nikita Khrushchev transferred jurisdiction from Russia, a move that was a mere formality until the 1991 Soviet collapse meant Crimea landed in an independent Ukraine.
That complicated history hangs over Ukraine's crisis.
"Crimea," said Pifer of the Brookings Institution, "is the flashpoint everybody needs to be watching."