DAMASCUS   (AFP/Reuters) - A bombing in the heart of Damascus killed at least 13 people Tuesday, as Russia banned its civilian planes from Syrian airspace after the crew of one reported coming under threat over the war-hit country.

US President Barack Obama, meanwhile, expressed concerns over the alleged use of chemical weapons by Syria’s regime in a telephone call with his Russian counterpart Vladimir Putin.

The bomb attack in the Damascus district of Marjeh came a day after Syrian Prime Minister Wael al-Haqi survived a car bombing in an upscale neighbourhood of the capital.

“The number of casualties in the cowardly terrorist blast targeting the commercial and historic centre of Damascus in the Marjeh district rose to 13 martyrs and more than 70 injured,” state television said, citing the interior ministry.

The Syrian Observatory for Human Rights reported “14 dead, including nine civilians and five members of the security forces, in a car bomb attack near the old interior ministry headquarters.”

Official television channels aired footage of smoke billowing over the site of the explosion, which blew out windows of cars, knocked off the bumpers of some and started fires.

The windows of the interior ministry building were blown out and a commercial complex, Burj Dismshiq, was devastated. Uniformed and plainclothes security forces could be seen running near the scene, as residents fled. “Internationally financed and supported terrorism committed a terrible massacre against civilians,” state television said.

Mutilated bodies could be seen in the street, and at least one body wrapped in a white sheet was laid out alongside an ambulance. “What mistake have we committed? I was going to work. Look at the bodies. Is this the freedom they want?” a bystander told state media.

Meanwhile, Iran said on Tuesday it regarded the alleged use of chemical weapons in Syria’s civil war as a “red line”, echoing major adversary the United States but saying Syrian rebels were the main culprit and not the Damascus government.

Aid organisation Oxfam urged the UN Security Council on Tuesday to help improve humanitarian access to war-torn Syria, saying more funds are needed as the “catastrophe worsens.”

As Oxfam chief Mark Goldring visited Syrian refugees in Jordan, the organisation called on the Security Council to press Damascus and rebels to help ensure aid reaches those most in need. “This could mean allowing aid to cross lines of control and cross-border from neighbouring countries, such as Jordan, Lebanon and Turkey,” Oxfam said in a statement.

“The world risks failing the people of Syria at a time when they most need our help. Responding to this crisis is now our number one priority,” it quoted Goldring as saying.

“Restrictions on access mean far too many vulnerable people are not getting the help they have a right to.”