DAMASCUS - Syria on Thursday came under intense pressure to allow UN inspectors to visit the site of an alleged chemical weapons attack near Damascus which the opposition says left hundreds dead and provoked revulsion around the world. A US official cited by the Wall Street Journal said there were "strong indications" that the government had carried out a chemical weapons attack, after the opposition reported more than 1,300 people killed by poisonous gases in rebel-held towns east of Damascus on Wednesday. Damascus has vehemently denied it unleashed chemical weapons, with a high ranking security source saying that to do so while a UN inspections team is working inside the country would be "political suicide". "All analysts say that it is not in our current interest to use chemical weapons while the commission is on the ground," he added.

France said it is seeking a reaction with "force" if allegations by the National Coalition, the main opposition group, of a massacre involving chemical weapons are confirmed.

"If it is proven, France's position is that there must be a reaction, a reaction that could take the form of a reaction with force," French Foreign Minister Laurent Fabius told BFM-TV, although he ruled out the use of ground troops.

There has been no immediate independent confirmation of the attack but videos and photographs posted online by activists showed scenes of people foaming at the mouth and of bodies laid out in long lines.

In one video posted on YouTube, children are seen receiving first aid in a field hospital, notably oxygen to help them breathe. Doctors appear to be trying to resuscitate unconscious children.

Another video showed what activists said was a case of hysteria following a chemical strike in the eastern suburbs. The authenticity of the videos has not immediately been verified.

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.

The Syrian Observatory for Human Rights reported that regime forces on Thursday bombed and shelled a number of rebel zones around Damascus where the Coalition alleged the attacks had taken place the previous day.

The Britain-based watchdog, which uses names and videos to document killings in the Syrian war, gave a toll of 170 people killed on Wednesday - 71 men, 34 women, 25 children and 40 rebel fighters.

Western governments demanded immediate access to the sites of the alleged attacks for a team of UN inspectors already in Syria to probe previous allegations of chemical weapons strikes.

The head of the UN inspection mission, Aake Sellstroem, was in talks with Damascus "on all issues pertaining to the alleged use of chemical weapons, including this most recent reported incident," a UN statement said.

White House spokesman Josh Earnest demanded the inspectors be given "immediate access to witnesses and affected individuals" and "the ability to examine and collect physical evidence without any interference or manipulation from the Syrian government."

Washington has previously described chemical weapons use as a red line that might prompt it to intervene militarily in Syria.

Turkish Foreign Minister Ahmet Davutoglu said Thursday that such a red line had already been crossed and called for international action. "We call on the international community in this situation where the red line was crossed long ago to intervene as soon as possible," he said in Berlin after talks with his German counterpart Guido Westerwelle.

The Wall Street Journal reported a senior administration official as saying the claims of a chemical weapons attack appeared to have some credibility.

"There are strong indications there was a chemical weapons attack - clearly by the government," the unnamed official said. "But we do need to do our due diligence and get all the facts and determine what steps need to be taken."

Human rights groups backed calls for access for the UN inspectors to the sites of the alleged attacks. Should the allegations be true, "the attacks would amount to war crimes," said Amnesty International's deputy director for the Middle East and North Africa Hassiba Hadj Sahraoui.

New York-based Human Rights Watch said that the descriptions its staff heard from witnesses are "consistent with the use of chemical nerve agents."

"A huge number of people in Ghouta (towns east of Damascus) are dead, doctors and witnesses are describing horrific details that look like a chemical weapons attack and the government claims it didn't do it," said Joe Stork, HRW's acting Middle East director.

The UN inspectors arrived in Damascus on Sunday with a strict mandate to investigate three sites for the alleged use of chemical weapons.

More than 100,000 people have been killed in Syria's 29-month war, the UN says. Millions more have been forced to flee their homes.

Meanwhile, Syrian forces blasted rebel zones near Damascus on Thursday, including areas where the opposition alleged loyalists had launched a chemical weapons attack a day earlier, a monitoring group said.

The air raids and shellings appear to be part of an all-out army offensive to rout rebels from strategic points surrounding the capital, activists said.

"Warplanes carried out several raids this morning on Khan al-Sheikh and Zamalka, wounding several people" and sparking fierce clashes, said the Syrian Observatory for Human Rights.

It said that other areas around the capital were also subjected to aerial bombardments, including Moadamiyet al-Sham, Daraya and Irbin.

"Zamalka and Moadamiyet al-Sham were also heavily shelled," it added.

Iran has rejected claims the Syrian regime, its chief regional ally, had deployed chemical weapons, saying if such an attack were proven rebels would be responsible, IRNA news agency said Thursday.

"If the information concerning the use of chemical weapons is accurate, very definitely they were used by terrorist groups... who have shown they will not hold back from committing any crime," Foreign Minister Mohammad Javad Zarif was quoted as saying, referring to rebels locked in a deadly war with President Bashar al-Assad's forces.