Ivan Sekretarev/Associated Press

WASHINGTON — President Obama issued a blunt warning to Moscow on Friday that "there will be costs" if Russia sends its troops into Ukraine, saying he is deeply concerned about reports of Russian military movements in the region.

Obama told a hastily convened White House press gathering that Russian military action would violate international law and "would be deeply destabilizing, which is not in the interests of Ukraine, Russia or Europe. It would represent a profound interference in matters that must be determined by the Ukrainian people."

Authorities in Ukraine closed airspace over the Crimean peninsula late Friday and reports indicated that multiple Russian transport planes had landed at a military air strip near Simferopol, Crimea's regional capital. Officials said flights to and from the commercial airport were canceled late Friday as well.

Ukrainian media reported disruptions "by unknown persons" of telephone and Internet communications, and said gunmen had surrounded a television station. Ukraine's acting president accused Russia of trying to seize the territory, a semi-autonomous region of Ukraine that is important to Russia for historical and strategic reasons.

Russia leases the Crimean port of Sevastopol, the longtime headquarters of its Black Sea fleet. The region's population is dominated by ethnic Russians.

The tension over Crimea could affect U.S. cooperation with Russia on the Syrian civil war, the international effort to negotiate a nuclear deal with Iran and other issues. The White House has been cautious regarding Ukraine in part because conflict with Russia could disrupt collaboration on other major problems.

Obama did not say what the United States will do — or can do — to head off Ukraine's threatened slide toward renewed civil conflict and a possible breakup as pro-Russia militants push for secession in Crimea. But he suggested that there would be some sort of international action if Russia intervened.

"Just days after the world came to Russia for the Olympic Games, it would invite the condemnation of nations around the world," Obama said. "And indeed, the United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine."

Russia is scheduled to host a meeting of the Group of Eight industrial nations in Sochi in June. That now may be in jeopardy.

Vice President Joe Biden spoke with Ukraine's prime minister by phone to promise U.S. support, White House advisors said. The president has directed his aides to coordinate with European allies and to communicate directly with Russian President Vladimir Putin's government, the advisors said.

Washington and its European allies have an untested relationship with the fragile new pro-Western government in Kiev, Ukraine's capital. They have even less influence over the armed men who have seized government buildings in Simferopol, or their presumed backers in the Kremlin, which is determined not to lose its only foreign naval base in Crimea.

"We're watching the unfolding of a nightmare scenario that people have worried about since the breakup of the Soviet Union" in 1991, said Andrew S. Weiss, an expert on Russia and Ukraine who served in the George H.W. Bush and Bill Clinton administrations.

"Putin's key goal has been to try to establish that 'Russia's back,'" said Weiss. "Now it looks like you're reckoning with a Russia that is acting … in a very dangerous way."

The West's greatest point of leverage, a promised multibillion-dollar bailout to help Ukraine's economy avoid collapse, faces resistance in Congress and in financially strained European capitals.

"There's no political will," said Eugene Rumer, who until December was the U.S. national intelligence officer for Russia and the region.

Acting Ukrainian President Oleksandr Turchynov said Russia was trying to seize territory in Crimea and "provoke us to a military conflict."

Turchynov's representative in Crimea, Sergei Kunitsin, later said in televised remarks that 13 Russian jumbo jets landed at Gvardeyskoye, near Simferopol, carrying an estimated 2,000 Russian paratroopers.

The moves appeared intended to demonstrate the Kremlin's determination to secure its port at Sevastopol, from which Russian naval might is projected to the world.

Kiev retains control over Ukrainian military forces in the western and central areas of the country, and even most Russian-leaning areas in the east have refrained from defying the new government.

But troops in Crimea may not be reliable in the face of the local population's rejection of Kiev's authority. With pro-Russia gunmen at airports and communications centers, it was unclear whether Kiev could bring in forces to challenge any Russian buildup.