US lawmakers warned of the consequences of Russian military action in the Middle East moments after high-flying Russian warplanes unleashed the "most violent and ferocious" airstrikes Syria has ever known in four years of civil war on Wednesday.

Russia's first-ever direct military assault in the region killed at least 33 civilians including children in rebel-held areas of Homs province, according to a Reuters report.

"We have been exposed to a wide range of weapons over the last five years, but what happened today was absolutely the most violent and ferocious, and the most comprehensive in the northern Homs countryside," said a doctor in the town of Rastan, one of the areas hit by the Russian jets, according to a Reuters report.

The doctor said the Russian warplanes flew at high altitude and emitted no noise before dropping their bombs and firing their missiles, catching everyone by surprise.

He said one of the bombs hit a house obliterating it, killing three children, their father and two others. "It was as if the house never was," said the doctor, who was one of those who helped treat wounded residents.

Russia said its air force struck Islamic State targets. However, Western military analysts and Syrian rebels noted that the airstrikes were focused on the city of Homs which is not known to be an ISIS stronghold.

"The northern countryside of Hama has no presence of ISIS at all and is under the control of the Free Syrian Army," Major Jamil al-Saleh of the Free Syrian Army told Reuters. This statement was confirmed by US officials and the Britain-based Syrian Observatory for Human Rights.

US lawmakers warned that Russian forces may hit both ISIS and Syrian opposition forces.

Sen. John McCain wasted no time in blaming the Obama administration for allowing a direct Russian military incursion in the Middle East, a first in the region.

"These airstrikes are indiscriminate in nature," McCain said, ridiculing Obama's "leading from behind" approach.

He said the new situation in the region has "now created a platform for a Russian autocrat to join with an Iranian theocrat to prop up a Syrian dictator."

"It did not have to be this way," McCain said. "This is the inevitable consequence of hollow words, red lines crossed ... and a total lack of American leadership. ... This is a very, very, very sad day for America and the world."

Rep. Adam Schiff, D-California, the top Democrat on the House intelligence committee, also warned of the consequences of Russia's military action.

"The use of Russian military force in Syria adds a troubling new development to a war effort already plagued with problems," he said in a statement. "... The Russian air campaign may be even more destructive if it targets moderate rebel forces fighting the Assad regime. Already, the growing Russian presence has thrown a life line to embattled dictator Bashar al-Assad, at a time when pressure on the regime and its supporters may have finally led to a negotiated end to the conflict."

He said "the increased longevity of the regime—made possible by this Russian intervention –will only prolong the civil war."

At the United Nations, US Secretary of State John Kerry rejected Russia's demand for US warplanes to stop flying over Syria to avoid getting targeted by Russian jets. Kerry said the Russian airstrikes will not stop coalition forces from completing their mission. "These strikes will continue," Kerry said.

At the same time, Kerry said the US would not object to Russian military action if its directed only at the ISIS and al Qaeda.

"We would have grave concerns should Russia strike areas where ISIL and Al Qaeda-affiliated targets ... are not operating," Kerry said, adding that the coalition's campaign against ISIS is not meant to support the Assad regime.