Revulsion over photos of child victims of an alleged chemical weapons attack in Syria left the White House facing new calls for a firm response Thursday.

Republican Senator John McCain warned President Barack Obama had given President Bashar al-Assad a "green light" to commit atrocities by failing to use military force to respond to previous attacks.

The White House said it was appalled by reports of an attack which rebels said killed 1,300 people outside Damascus, and renewed calls for Syria to allow a UN probe.

McCain has been a frequent critic of Obama's reluctance to commit US military forces to protect civilians in Syria, and bemoaned the horror of the latest attack.

"When the president of the United States says that if he uses these weapons that it would be a, quote, 'red line and a game-changer,' (Assad) now sees that as a green light," he told CNN.

"The word of the president of the United Sates can no longer be taken seriously, as it isn't throughout the entire region."

McCain said that in a "couple of days" US air power could take out Syrian air force runways and up to 50 aircraft being used to dominate battlefields against rebels.

"We can supply the right kind of weapons to rebels, establish a no-fly zone by moving Patriot missiles up to the border.

"This can be done very easily."

White House spokesman Josh Earnest said the administration was horrified by the latest reports.

"The images that we have seen are nothing short of horrifying," he said.

He repeated a US call for Syria's government to permit a United Nations team already in the country to probe previous alleged chemical attacks to examine the new claimed outrage.

Shocking pictures of dead children blanketed the front pages of US newspapers and were shown on cable news shows Thursday, subjecting the White House Syria policy under new scrutiny.

The Wall Street Journal cited an unnamed official as saying there were "strong indications" that Syrian government forces had carried out a chemical weapons attack.

The White House declined to confirm that statement, and has said it is seeking to verify the reports.

Damascus has vehemently denied it unleashed chemical weapons on rebel-held area outside Damascus.

If confirmed, the attack would be the largest scale use of chemical weapons since Saddam Hussein's Iraqi forces attacked the Kurdish town of Halabja in 1988.

On Wednesday, the White House refused to say that if the attack was proven it would represent another violation of US "red lines" for Syria

Officials said on Wednesday that Washington did in fact respond to the previous use of chemical weapons in Syria by offering military aid to rebels for the first time.

The top US military officer General Martin Dempsey also warned the use of force in Syria could end up embroiling Washington in an open-ended war in the country.