President Vladimir Putin on Saturday won approval from Russia's upper house of parliament, the Federation Council, to send Russian troops into Ukrainian territory.

Putin's request was approved unanimously. Speaker Valentina Matviyenko also ordered the Council's foreign affairs committee to ask Putin to recall the Russian ambassador from the United States.

British Foreign Secretary William Hague has urged his Russian counterpart to de-escalate the situation in Ukraine's restive Crimea peninsula, ahead of a trip to Kiev on Sunday.

"Have spoken to Foreign Minister (Sergei) Lavrov to call for de-escalation in Crimea and respect for sovereignty and independence of Ukraine," Hague said in a Twitter message on Saturday.

A referendum to determine whether residents in Ukraine's flashpoint peninsula of Crimea want greater autonomy has been pushed forward to March 30, the spokeswoman of the region's newly-chosen prime minister Sergiy Aksyonov said Saturday.

The vote had originally been planned for May 25, on the same day as presidential elections set by parliament following the ouster of the pro-Moscow President Viktor Yanukovych.

Polish Foreign Minister Radoslaw Sikorski cut short a visit to Iran on Saturday to handle the deepening crisis in Crimea after pro-Russian gunmen seized the government and parliament buildings.

"I have to shorten my visit to Iran due to the developments in Crimea and return to Poland," Sikorski told a joint news conference with his Iranian counterpart Mohammad Javad Zarif.

Russia has sent "several hundred" troops into Ukraine's Crimea region, U.S. defense officials said Friday, after Kiev called on Moscow to withdraw its forces from the peninsula.

The comments marked the first confirmation from President Barack Obama's administration that Russia had launched an incursion into Crimea.

President Barack Obama is warning Russia "there will be costs" for any military maneuvers it launches in Ukraine, a move U.S. and Ukrainian officials say they believe is already underway.

Officials say Obama may retaliate by canceling a trip to Russia this year for an international summit and could cut off trade discussions with Moscow. But it's unclear whether those moves will have any impact on Russia's strategy on Ukraine, which is at the center of what many see as tensions between East and West.

Russian aircraft carrying nearly 2,000 suspected troops have landed at a military air base near the regional capital of the restive Crimean peninsula, a top Ukrainian official said Friday, accusing Moscow of an "armed invasion."

"Thirteen Russian aircraft landed at the airport of Gvardeyskoye (near Simferopol) with 150 people in each one," Sergiy Kunitsyn, the Ukrainian president's special representative in Crimea, told the local ATR television channel, adding the air space had been closed.

Kiev's main airport said Friday that flights to the restive Crimean capital of Simferopol, where gun-toting men have encircled the airport, have been cancelled as tensions escalate on the peninsula.

A spokeswoman who did not give her name said flights for Friday evening and Saturday morning had been cancelled because "the situation in Simferopol does not allow them to accept flights".

Russia said Friday that it would fast-track passports for members of Ukraine's elite Berkut riot police force after the unit was disbanded by Kiev's new authorities, and issue them in the main Crimean city of Simferopol.

"The Russian consulate in Simferopol has been tasked with taking all the necessary measures to urgently start issuing Russian passports to Berkut officers," the Russian foreign ministry said on its Facebook page."

Russia is ready to help Ukraine as it seeks to stave off economic collapse, U.S. Secretary of State John Kerry said Friday after talking with his Russian counterpart.

Russian Foreign Minister Sergei Lavrov indicated during a Friday phone call "that they are prepared to be engaged and be involved in helping to deal with the economic transition that needs to take place at this point," Kerry told reporters.