Pro-Russian rebels shot down two Ukrainian helicopters Friday, killing two airmen and wounding others.

The rebels shot down the helicopters as troops tightened their hold on the separatist town of Slavyansk. Moscow accused Kyiv of launching a "criminal" assault, Voice of America reports.

Ukrainian forces are continuing military operations, but have made limited advancements. Regardless, Russian President Vladimir Putin's spokesman accused Kyiv of firing on civilians from the air in a "punitive" operation that dashed all hopes of a peace plan.

The Kremlin spokesman said Russia was "extremely worried" about Russians in the city, and said they sent an envoy to help free German and other foreign hostages.

Moscow currently has tens of thousands of troops on the Ukraine border, saying they will invade if they need to protect Russians.

One helicopter opened fire before dawn Friday in Slavyansk, which is the most heavily fortified area of pro-Russian separatists in eastern Ukraine. Ten hours later, the city was quiet, with shops closed and armed separatists patrolling the streets.

Ukrainian forces moved closer to the suburbs, but rebels still control most of the city.

On Thursday, the pro-Russian militants seized control of a rail control center for the Donetsk region. They cut off power, which nearly stopped train traffic.

Kyiv said the firing down of Ukrainian helicopters is evidence that Russian forces are in the city, but Moscow continues to deny having any Russian forces on the ground.

The Kremlin says there are threats to civilians ahead of the vote rebels aim to hold on May 11 to break with Kyiv, similar to the vote held in Crimea before Moscow annexed it in March.

About 100 people gathered Friday outside the city hall in Slavyansk, saying they want Putin to send troops to help them.

Businesswoman Tamara Voshchanaya said: "What can you think when the sound of cannon makes you jump out of bed, when helicopters are flying over and shooting at our guys?"

"We are ready to stand firm, we will not abandon the guys. Give us weapons, and we, too, will go against the Right Sector," she said, referring to a Ukrainian nationalist group.

The European Union said it is watching the events with concern. Kyiv is not a member of NATO, and Western leaders said they will not fight to defend Ukraine.

U.S. Defense Secretary Chuck Hagel said Friday that NATO's European members need to increase defense spending to defend Ukraine.

''We must see renewed financial commitments from all NATO members,'' Hagel said in excerpts of a speech that will be delivered at the Wilson Center Friday.

The Ukrainian Defense Ministry said in a statement that the two Mi-24 attack helicopters were shot down by anti-aircraft missiles while on patrol overnight.

Ukrainian officials said that one airmen was taken prisoner. A third helicopter that was carrying medics was also hit, and a serviceman wounded.

Russia blamed the Ukrainian government, which took power two months after pro-Western protests forced the elected Ukrainian president to flee to Russia.

Putin recently sent an envoy, Vladimir Lukin, to southeast Ukraine to negotiate the release of European military observers that are being held by rebels, but Lukin has not been heard from since the operation began.

Under the April Geneva Accord between Russia, Ukraine, the United States and the European Union, separatists were supposed to lay down arms and vacate the public buildings they have seized in about a dozen towns. However, they have not relinquished control.

Interior Minister Arsen Avakov said in a statement: "The goal of our anti-terrorist operation and, at the same time, our demands to the terrorists are simple: Free the hostages, lay down weapons, vacate administrative buildings and get municipal infrastructure back to normal."

People are urged to stay indoors, and Ukrainian forces have orders not to fire on residential buildings.