The west needs to calm its rhetoric about military intervention in Syria after reports that up to 1,400 people have died in a suspected chemical weapons attack, Lord Malloch-Brown, a former deputy secretary-general of the UN, has said.

As France called for "reaction with force" if the massacre is confirmed and Turkey said "all red lines" had been crossed, the ex-Foreign Office minister urged the international community to step back from the threat of military action.

Malloch-Brown called instead for a measured reaction as UN officials said the alleged attacks in Damascus were a "serious escalation", with 35 member states demanding that Syria let inspectors assess whether chemical weapons have been used.

Speaking on BBC Radio 4's Today programme, Malloch-Brown said the Syrian president, Bashar al-Assad, was "badly misreading the west" if he thought he had a "blank chequebook to commit any kind of atrocity" without provoking a reaction.

However, he urged more diplomatic talks with Russia and China to find common ground for exerting pressure on Syria, which must include allowing access for UN inspectors.

"We need to bring down the rhetoric from red line or military intervention to try to find common ground that we can agree on with the Russians and the Chinese, to establish the facts," he said. "Was it an astonishing government attack of extraordinary brutality against civilians or was somebody else responsible? And then move on from there.

"But at the moment this is already morphing into, 'What next?' and that of course immediately raises the hackles of the west's opponents on the security council. Without an inspection it's going to be a series of unproved charges and counter charges between the different sides."

He said there were similarities with the Balkans, where diplomats "dug their heels in against intervention" and then President Bill Clinton took an "extraordinary turn of direction" despite having been a "reluctant interventionist".

Syria has acknowledged launching a major offensive in rebel-held districts but claims the scenes of carnage have been faked "to serve the propaganda of the terrorists". Medics, opposition fighters and political leaders have put the death toll somewhere between 200 and 1,400 but none of the figures have been independently verified.

UN inspectors have been allowed into Syria to look into earlier claims of chemical weapons atrocities. They have been staying just a few miles away from the scenes of carnage, but they have not been allowed to assess the fresh claims that chemicals have been used.

"What needs to happen is for the security council to unequivocally focus on access for these inspectors," Malloch-Brown has said. "No report they do on either historic incidents is going to have any credibility if this goes uninvestigated."

Barack Obama has previously said the use of chemical weapons in Syria is a "red line". On Wednesday night, the White House said it was "deeply concerned" about the latest reports.

Downing Street sources said Britain's strategy is to push for UN inspectors to attend the scene. Britain called the emergency UN security council meeting on Wednesday, with William Hague, the foreign secretary, condemning a "shocking escalation in the use of chemical weapons in Syria" if the reports are verified.

"I hope this will wake up some who have supported the Assad regime, to realise its murderous and barbaric nature," he said.

However, Russia has already come to Syria's defence, saying the reports suggest a "pre-planned provocation".