Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

MOSCOW — Russia began carrying out strategic airstrikes in Syria on Wednesday, warning the U.S. to steer clear of the country's airspace as its warplanes joined in the fight against ISIS.

But the U.S. State Department said Moscow's request would be ignored, adding that American jets would continue to fly missions as part of a separate air campaign to root out the militants. Secretary of Defense Ash Carter compared the bombing campaign to "pouring gasoline on a fire" and said Russia's bombardments didn't appear to be targeting areas held by ISIS forces.

Secretary of State John Kerry said the U.S. would have "grave concerns should Russia strike" areas where ISIS wasn't operating.

"Strikes of that kind would question Russia’s real intentions fighting ISIL or protecting the Assad regime," he said, using another name for ISIS.

Later on Wednesday, Carter told reporters that Russia was wrong to back Assad. "Fighting ISIL without pursuing a parallel political transition only risks escalating civil war in Syria," he said.

Russian Ministry of Defense spokesman Igor Konashenkov confirmed his nation's warplanes were "conducting an air operation with surgical strikes on ground targets of the terrorist group ISIS" in key Middle East ally Syria.

He added that "ISIS’s military equipment, communication hubs, transportation, and ammo and fuel depots" were being targeted.

A U.S. defense official told NBC News that Russia conducted airstrikes in Talbisah, near Homs, and in the Al-Ghab Valley, near Hama. The official said the targets of the strikes have not been confirmed.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Maria Zakharova, a spokeswoman for the Foreign Ministry, dismissed reports Wednesday that Russia targeted non-ISIS opposition or killed civilians in Syria.

Earlier Wednesday, Russian lawmakers voted unanimously to allow President Vladimir Putin to order airstrikes in Syria, where Moscow has deployed fighter jets and other weapons in recent weeks.

Putin said the only way to fight "terrorists" in Syria was to act preemptively.

ISIS is more powerful than ever and leaders at UN terror summit don't know what to do about it @NBCNightlyNews — Andrea Mitchell (@mitchellreports) September 29, 2015

State Department spokesman John Kirby said that a Russian official in Baghdad warned the U.S. Embassy personnel that the anti-ISIS missions would begin Wednesday.

Kirby added: "He further requested that U.S. aircraft avoid Syrian airspace during these missions ... The U.S.-led coalition will continue to fly missions over Iraq and Syria as planned and in support of our international mission to degrade and destroy ISIL."

According to the U.S. defense official, the Russians provided less than an hour's notice that the bombings were about to begin.

U.S. officials had said in recent days that the Russians were flying reconnaissance missions over Syria without dropping bombs to familiarize themselves within the area. That was taken as an indication that they were about to begin airstrikes, they added.

Related: Richard Engel Analysis: How Russia Is Aiming for Syria Checkmate

Putin said that his country would support Assad without participating in ground operations, according to the Interfax news agency.

He said the airstrikes by Russia came after a request by the Syrian administration and were being carried out "on the basis of international law," Interfax reported.

Sergei Ivanov, the Kremlin's chief of staff, said Russia's missions would be limited and not open-ended. He precluded the use of ground troops.

"The military aim of our operations will be exclusively to provide air support to Syrian government forces in their struggle against ISIS," Ivanov added.

The airstrikes mark Russia's biggest play in the region since the 1991 collapse of the Soviet Union.

A U.S.-led coalition has already been bombing ISIS in Iraq and Syria, but Putin derided Washington's efforts to end the Syria war at the United Nations on Monday. He suggested a broader and more coordinated coalition was needed to defeat the militants.

Putin’s endorsement of an international coalition helps put him in a position to help prop up Assad, an ally in the region, and could help Moscow play a pivotal role in choosing the Syrian leader’s successor should he leave office, said David Rothkopf, a former Clinton administration official and editor of Foreign Policy Group, a collection of foreign policy publications.

“It’s a deliberately calculated step by Putin to say Obama and the West today aren’t doing enough and we should step up and fill the void,” Rothkopf said.

Sen. John McCain, R-Ariz., chairman of the Senate Committee on Armed Services, took to the floor on Wednesday and placed the blame for Russia's increased presence in the region squarely with the Obama administration.

"President Obama has sounded retreat across the Middle East," said McCain who has long been a critic of the administration's policy in the region. He added that a series of "decisions or non-decisions" have led to a situation where the U.S. has "confused our friends (and) encouraged our enemies."

"Into the wreckage of this administration’s Middle East policy steps Vladamir Putin," McCain said.