President Barack Obama has ordered US spy agencies to urgently probe claims that Syrian forces launched a horrific chemical attack on civilians, including children, aides said Thursday.

Officials said they could not yet be sure the deadly arms had killed as many as 1,300 people outside Damascus, even though US ally France said it was likely they were used.

The Obama administration is facing rising political pressure for a tough response to the attack, which could have again infringed US "red lines" against chemical weapons use by President Bashar al-Assad's regime.

State Department spokesman Jen Psaki said that Obama had asked intelligence agencies to urgently evaluate claims by Syrian rebels about the attack.

"Right now, we are unable to conclusively determine CW (chemical weapons) use," Psaki said.

But she said the administration had been focused continually since reports of the attack broke on Wednesday on efforts to "nail down the facts."

Secretary of State John Kerry discussed the episode with key counterparts, including Syria's main opposition leader Ahmad Assi Jarba and French Foreign Minister Laurent Fabius.

Kerry also called top officials at the European Union, the United Nations and in Jordan, Qatar and Turkey.

"If these reports are true, it would be an outrageous and flagrant escalation of (the) use of chemical weapons by the regime," Psaki said.

Obama has a range of options at his disposal and would discuss a response with his top national security aides, Psaki said.

The president has been loath to order US military action to protect civilians in Syria, fearing being drawn into a vicious civil war, just soon after he extracted US troops from Iraq.

But revulsion over video footage and photos of dead children blanketing the US media has reopened the debate about a policy Obama allies see as prudent but critics brand as weak.

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

"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 States can no longer be taken seriously, as it isn't throughout the entire region."

The White House concluded earlier this year that Syria had crossed Obama's red line against the use of chemical weapons, but opted not to take military action.

Instead, it decided to directly arm selected Syrian rebel groups battling Assad -- but has declined to publicly specify the extent of the support.

McCain said he wants to see more robust action.

He 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," he said.

"This can be done very easily."

America's top military officer General Martin Dempsey has said however that Washington risks being dragged into an open-ended war in Syria if it chooses a military intervention.

White House spokesman Josh Earnest said the administration was appalled 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.

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.

French President Francois Hollande contributed to rising heat on the White House when he said it was "likely" Assad forces had deployed chemical weapons in the attack on a rebel-held Damascus suburb.

Fabius had earlier said that the world should respond "with force" if it was proven the massacre was perpetrated with chemical agents. But he ruled out the use of ground troops.

Damascus has vehemently denied it unleashed chemical weapons in the attack.

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.

Chemical weapons, developments in Syria
Nicosia (AFP) Aug 22, 2013 - Below are events that have led to charges by the Syrian opposition that government troops unleashed chemical weapons on civilians near Damascus this week.

--2012--

- July 23: The Syrian regime acknowledges for the first time that it has a chemical weapons stockpile and threatens to use it in case of a Western military intervention, but never against its own people.

- August 20: US President Barack Obama says the United States would regard any recourse by Damascus to its deadly arsenal as crossing a "red line".

- December 3: NATO warns Syria against using chemical weapons, as a US official says that Syria has begun mixing chemicals that could be used to make sarin gas, a deadly nerve agent.

- December 24: Russian Foreign Minister Sergei Lavrov says it would be "political suicide" for Syrian President Bashar al-Assad to use chemical weapons against the armed opposition, and says he does not believe he would.

--2013--

- January 30: The Israeli air force hammers a military complex near Damascus suspected of holding chemical agents, saying it fears their transfer to Lebanese Shiite militant group Hezbollah, according to a US official.

- March 19: The government and rebel forces accuse each other of using chemical weapons for the first time.

- March 20: Obama warns the Damascus regime any use of chemical weapons against civilians would be a "game-changer".

- April 8: UN Secretary General Ban Ki-moon says a UN inspection team is in nearby Cyprus and ready to deploy. Syria rejects the mission as proposed by Ban.

- April 25: The White House says that Syria has probably used chemical weapons against rebel forces on a "small scale," while emphasising that US spy agencies are still not 100 percent sure.

- May 10: US Secretary of State John Kerry says there is "strong evidence" that the regime has used chemical weapons against rebels.

- May 18: Assad denies that his forces have used chemical weapons.

- June 4: UN investigators say they have "reasonable grounds" to believe that both sides in Syria have employed chemical weapons, on four occasions. France says it "is clear" that Assad's regime is using sarin gas.

- June 13: The White House says a "red line" has been crossed and accuses the Syrian regime of using chemical weapons.

- June 14: The Kremlin says the US accusations are "unconvincing". Damascus calls them "lies". Britain says it agrees with the US assessment. NATO says Damascus must let the UN investigate.

- July 9: Russia's ambassador to the UN says Moscow has proof that rebels used sarin gas in the northern town of Khan al-Assal on March 19. The White House says it has seen no such proof.

- July 23-24: Two senior UN chemicals weapons experts hold talks in Damascus.

-August 14: UN announces the Syrian government has formally accepted "the modalities essential for cooperation to ensure the proper, safe and efficient conduct" of an inspection mission.

It says the inspectors led by Swedish arms expert Aake Sellstroem will investigate three sites of alleged chemical attacks for a period of at least two weeks.

- August 18: UN inspectors arrive in Damascus.

- August 21: The Syrian opposition accuses the regime of killing more than 1,300 people with chemical weapons near Damascus, and posts grim video images to the Internet. The government denies using chemical weapons.

- August 22: The international community and human rights organisations call on UN experts in Syria to probe the charges of a chemical weapons attack on civilians.