A bombing in the heart of Damascus killed at least 13 people Tuesday as US President Barack Obama warned against rushing to act on allegations Syria's regime used chemical weapons on its own people.
The devastating attack in Marjeh district came a day after Syrian Prime Minister Wael al-Haqi survived a car bombing in an upscale neighbourhood of the capital.
Hezbollah chief Hassan Nasrallah acknowledged, meanwhile, that forces of his powerful Lebanese Shiite militia were inside Syria fighting alongside President Bashar al-Assad's loyalists.
Rebels had been "preparing to capture villages inhabited by Lebanese," so it was "normal to offer every possible and necessary aid to help" regime forces and the Lebanese, he said.
Nasrallah also said Assad has "true friends in the region who will not allow Syria to fall into the hands of the United States, Israel and 'takfiri' groups," a reference to Sunni Muslim rebels.
Obama warned against rushing to judgement on the use of chemical weapons in Syria but said proof of their use would trigger a "rethink" of his reluctance to use military force.
"I've got to make sure I've got the facts. That's what the American people would expect," he told a news conference.
"If I can establish in a way that not only the United States but also the international community feel confident in the use of chemical weapons by the Assad regime, then that is a game changer," Obama warned.
"And what we now have is evidence that chemical weapons have been used inside of Syria, but we don't know how they were used, when they were used, who used them."
In New York, Syria stuck to its refusal to give UN experts unconditional access for a chemical weapons probe, even as it alleged rebels used them near the northern city of Idlib.
Back in Damascus, state media blamed "cowardly terrorists" for the bombing that it said killed 13 people and wounded, using its term for the anti-Assad forces.
The Syrian Observatory for Human Rights reported 14 dead, including nine civilians and five security forces members, in the attack near the old interior ministry.
Official television showed smoke billowing over the scene, with the ministry's windows blown out, a business complex devastated and cars damaged.
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Mutilated bodies could be seen in the street, and at least one body wrapped in a white sheet was laid out alongside an ambulance.
Uniformed and plainclothes security forces could be seen running near the scene, as residents fled.
"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.
On Monday, a car bomb targeted Haqi's convoy as it passed through Mazzeh neighbourhood, killing one of his bodyguards and five other people, said the Observatory.
Halqi, appointed premier in August 2012 after his predecessor Riad Hijab defected to the opposition, is the latest in a growing list of regime officials to be targeted for assassination.
As bloodshed continued unabated, the White House said Obama raised "concern over Syrian chemical weapons" in a telephone call with Russian President Vladimir Putin.
Obama is under pressure because he warned last year the use or movement of chemical weapons by Assad's forces would cross a US "red line".
Key political players are saying his credibility is on the line, but the White House is seeking precise intelligence into how and when such weapons may have been used.
Washington has been deeply frustrated that Russia has blocked tougher action in the UN Security Council, including sanctions, against its long-time ally Syria.
In Moscow, the air transport agency banned civilian planes from Syrian airspace after the crew of a charter plane flying from Egypt to the Russian city of Kazan on Monday said it had come under threat over the war-hit country.
The plane carrying 159 passengers said it detected "signs of combat actions" from Syria, officials said without providing further details.
A Moscow source told Interfax news agency assailants fired two land-to-air missiles at the plane, which did not suffer any damage.
On the ground, the Observatory said air raids on Tuesday killed 15 people close to Mennegh airport near Aleppo as well as the Jubar area of Damascus, and parts of Homs, Raqa and Latakia provinces.
Jordan warned the UN Security Council of the "crushing weight" of hosting more than 500,000 Syrian refugees. The UN expects their number to swell to 1.2 million refugees by the year's end.