(CNN) -- Two leading Republican lawmakers said Tuesday that reports of chemical weapons use in Syria, if true, mean the president's threshold for involvement in the country's civil war has been met.
Sens. John McCain and Lindsey Graham, who have led calls in Congress for greater U.S. involvement in Syria, issued a joint statement saying the attacks, if verified, would demand a response from President Barack Obama.
"If today's reports are substantiated, the President's red line has been crossed, and we would urge him to take immediate action to impose the consequences he has promised," the senators wrote.
Syrian state media cited government figures Tuesday who accused rebels of a deadly chemical weapons missile attack in the town of Khan al-Asal, in Aleppo province. Rebels rebuffed the claims and blamed the regime.
The town of Ateibeh, in eastern Damascus, also endured "fierce shelling with chemical rockets," an opposition group said. An unknown number of casualties were reported.
The attacks have not been confirmed by western governments, including Britain and the United States. But McCain and Graham said if they were true, a series of U.S. actions were necessary to prevent further violence in Syria.
"That should include the provision of arms to vetted Syrian opposition groups, targeted strikes against Assad's aircraft and SCUD missile batteries on the ground, and the establishment of safe zones inside Syria to protect civilians and opposition groups," McCain and Graham continued in their statement. "If today's reports are substantiated, the tragic irony will be that these are the exact same actions that could have prevented the use of weapons of mass destruction in Syria."
In the two years since Syrian rebels began trying to drive Assad from power, the United States has waded into the conflict incrementally. The U.S. government formally recognized the leading Syrian opposition coalition in December 2012, though stopped short of saying the U.S. would join other nations like Qatar in providing the rebels with weapons.
Obama laid out his barometer for greater action in Syria during a White House press conference in August, saying that "a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized."
"That would change my calculus," Obama said. "That would change my equation,"
White House Chief of Staff Denis McDonough explained Tuesday that if the reports of chemical weapons being used in Syria are validated, the Obama administration would take new action.
"If this is substantiated, it does suggest.this is a game changer and we'll act accordingly," McDonough said on CNN's "The Lead with Jake Tapper."
"I'm not going to get into hypotheticals, but this is something we take very, very seriously. We had teams up working on this overnight, as you might suspect, and we'll continue to trace this," he continued.