Crimean authorities said on Friday they had banned the leader of the peninsula's pro-Kiev Tatar community from flying to the Kremlin-controlled territory, claiming he wanted to destabilize the situation there.

Ukranian lawmaker Mustafa Dzhemilev, who is hugely respected by the Crimean Tatar minority, had attempted to fly to the peninsula's main city of Simferopol via Moscow, said a statement from the local authorities.

Russia is seeking to test NATO and members of the transatlantic alliance must increase military spending in the face of Moscow's challenge, U.S. Defense Secretary Chuck Hagel warned Friday.

The 28 members of NATO have responded to Russia's intervention in Ukraine with "resolve," Hagel said, "but over the long term, we should expect Russia to test our alliance's purpose, stamina, and commitment."

The United States and Germany delivered a firm warning to Russia on Friday that it would face direct and painful economic sanctions if Ukrainian elections later this month are disrupted.

U.S. President Barack Obama and German Chancellor Angela Merkel delivered the statement after talks at the White House appeared to bring the prospect of "sectoral" economic sanctions closer than ever before.

Three teams of Western journalists covering an assault by Ukraine's military on the flashpoint town of Slavyansk on Friday were seized by highly strung rebels and held for hours -- in some cases roughed up.

U.S. reporters for CBS television news and the online site Buzzfeed, and a team with Britain's Sky News were grabbed at rebel-held checkpoints as they tried to enter the town.

President Vladimir Putin's spokesman said Friday Ukraine's offensive against pro-Russian rebels in the east dealt a final blow to a peace deal on the crisis, adding a Moscow envoy had been dispatched there to help negotiate the release of kidnapped OSCE monitors.

"While Russia is making efforts to de-escalate and settle the conflict, the Kiev regime moved combat air forces against peaceful settlements, began a reprisal raid, essentially finishing off the last hope for the feasibility of the Geneva accords," Dmitry Peskov was quoted as saying by Russian news agencies.

Ukraine on Friday launched a military assault on the flashpoint town of Slavyansk, raising the stakes in the showdown with Russia, which has vowed "catastrophic consequences" if Kiev stepped up operations.

Insurgents shot down two army helicopters, killing two servicemen, including a pilot, as the army tightened its noose around the rebel-held town of 160,000 people.

Some 60,000 people waving Russian flags and brandishing posters of President Vladimir Putin took to the streets Thursday in Simferopol, capital of the Crimean peninsula annexed by Moscow in March.

The marchers carried banners reading: "We are Russia" and "Putin is our president", said an AFP reporter on the scene who estimated the size of the crowd.

A crowd of some 300 pro-Russian militants attacked the prosecutor's office in the eastern Ukrainian city of Donetsk on Thursday, Agence France Presse reporters on the scene said.

The mob hurled rocks and Molotov cocktails at around 100 riot police defending the building, who responded with stun grenades and tear gas, in the latest unrest to hit the crisis-wracked eastern part of Ukraine.

The International Monetary Fund approved a $17 billion aid deal for Ukraine on Wednesday, even as Kiev fought to prevent pro-Moscow separatists from grabbing another chunk of the country.

Greenlighting a rescue program for an interim government which took power after an uprising two months ago, IMF chief Christine Lagarde said it was crucial to strengthen Kiev's economy.

Ukraine has arrested the Russian defense attache in Kiev and decided to bring back military conscription with immediate effect to deal with a spreading pro-Moscow insurgency in its east.

According to a decree issued Thursday by interim president Oleksandr Turchynov, the country was bringing back military conscription "given the deteriorating situation in the east and the south ... the rising force of armed pro-Russian units and the taking of public administration buildings ... which threaten territorial integrity."