Obama sends out a warning to Assad

US President hints that he will send in troops if Syrian leader turns to chemical weapons

The use of chemical or biological weapons in Syria would constitute a "red line" for the United States, Barack Obama warned the Syrian President Bashar al-Assad yesterday.

Mr Obama told reporters at an unscheduled White House briefing that the use or deployment of such weapons was of high concern to both the US and Israel. "That would change my calculus," he said, noting that he had not "at this point" ordered US military engagement in Syria. He spoke as regime tanks and helicopter gunships launched an intense assault on several Damascus suburbs, free from the gaze of international observers, while Turkey called on the UN to create a "safe zone" inside Syria as it struggles to deal with the influx of tens of thousands of refugees.

An upsurge in violence in recent weeks has accelerated the flow of civilians fleeing the bloodshed but there are fears that yesterday's departure of the last members of the once 300-strong United Nations monitoring mission could encourage both sides to act with even further impunity.

As the final observers flew out of the capital on the second day of the Muslim festival of Eid al-Fitr, activists reported what they described as a particularly brutal assault on several areas of the capital where rebels have a strong presence. The Local Coordination Committees said 43 of the 84 people it had counted dead by yesterday afternoon had been killed in the Damascus suburbs. A video purporting to show the aftermath of shelling from a helicopter in Daraya showed men racing towards smoking buildings to look for wounded. Two bodies lay mangled in the street.

Mouadamiya was also attacked by tanks and, later, helicopters, activists said. The neighbourhood has changed hands several times during the uprising, but has largely been under rebel control since the regime focused its fight on the northern city of Aleppo.

The battle there has pushed more refugees over the Turkish border, and its Foreign Minister, Ahmet Davutoglu, said the country "will run out of space" if the number there surpasses 100,000. Turkey has approximately 70,000 Syrian refugees in its camps and crossed the border into Syria to deliver aid for the first time over the weekend. "We should be able to accommodate them in Syria," he told the Hurriyet newspaper. "The United Nations may build camps in a safe zone within Syria's borders." The newly documented use of fighter jets to launch air raids on civilian areas has renewed talk of a no-fly zone on the Turkish border.

However, if forwarded at the UN Security Council, such a proposal is also is bound to be blocked by Russia and China, who argue it is a pretext for military intervention.

Mr Davutoglu hoped a decision on the safe zone would be reached on 30 August, but many Syrians have lost hope for action by the international community. "It's just talk as usual but nothing will happen on the ground," said activist Mohammed Saeed.