World leaders have reacted with shock over alleged chemical attacks in Syria as UN officials describe the incident as a "serious escalation".
Deputy Secretary-General Jan Eliasson spoke after an emergency UN Security Council meeting over Wednesday's attack.
The UK and 36 other countries have formally requested urgent access to the site of the gas attack in Damascus in a letter addressed to the United Nations secretary general Ban Ki-moon. The opposition claims more than 1,300 people died in the attack by Bashar al-Assad's forces using chemical weapons in Ghouta.
A Foreign Office spokesman said: "The UK along with 36 other countries has written to the secretary general formally referring this incident to the UN and calling for the UN team to be granted the necessary access to enable their investigation into these latest allegations as a matter of urgency."
Following a meeting with Turkish foreign minister Ahmed Davutoglu, Foreign Secretary William Hague said they had discussed working together towards a political solution.
He said: “We discussed the situation in Syria and appalling reports that hundreds of people have been killed in a chemical weapons attack and airstrikes on rebel-held areas near Damascus.
”We agreed that the Syrian government must allow immediate access to the area for the UN team currently investigating previous allegations of chemical weapons use.
Mr Elliasson, speaking with reporters after the meeting, said UN weapons inspectors already in Syria would not be able to investigate the incident until consent was given by the Syrian Government.
The inspectors had arrived on Sunday after months of negotiations with the regime to investigate three occasions where chemical agents have, allegedly, been used in the past.
The Syrian Network for Human Rights, an opposition monitoring group, put the number at 587 dead but the organisation warned that the death toll was still “initial”.
Extensive amateur video and photographs appeared on the internet showing victims choking, some foaming at the mouth.
Doctors interviewed described symptoms they believe point to sarin gas, one of the agents Western powers accuse Damascus of having in an undeclared chemical weapons stockpile.
The Assad regime has denied the allegations, accusing “terrorists and their supporters” in the international media of disseminating false propaganda.
French Foreign Minister Laurent Fabius has raised the possibility of the use of force in Syria if it is proven that Bashir Assad's regime used chemical weapons.
Fabius made the remark on Thursday, a day after the U.N. Security Council called for “a thorough, impartial and prompt investigation” of the latest allegation of chemical weapons use in Syria, outside Damascus. Opposition forces and activists say at least 136 people, including many children, were killed.
Fabius says if the allegations are proven, “we need a reaction by the international community .... a reaction of (military) force.”
He excluded boots on the ground as an option, “but a reaction that can take a form, I don't want to be more precise, of force.”
Mr Davutoglu also spoke today, announcing that “all red lines” had been crossed in Syria. He criticised international inaction after the opposition accused Syrian government forces of gassing hundreds in a chemical weapons attack.
“All red lines have been crossed but still the UN Security Council has not even been able to take a decision. This is a responsibility for the sides who still set these red lines and for all of us,” Davutoglu told reporters in Berlin.
China also released a statement, asking that the UN inspection team maintained an “objective, impartial and professional stance” if they are granted access to the site.
“China's position is very clear. It does not matter what side in Syria uses chemical weapons, China resolutely opposes it,“ the Foreign Ministry told Reuters.
”At present, the U.N's chemical weapons inspection team for Syria is on the ground beginning its investigations, and (China) hopes that the team fully consults with the Syrian government and maintains an objective, impartial and professional stance, to ascertain what really happened."
Israel was the most recent country to speak out, saying it believed Syrian forces had used chemical weapons and accusing the world of turning a blind eye to such attacks.
"The world condemns, the world investigates, the world pays lip service," Strategic Affairs Minister Yuval Steinitz said on Israel Radio.
"Nothing tangible or significant has been done in the past two years to halt Assad's incessant massacre of his citizens," he said.
Additional reporting by Reuters