A bombing in the heart of Damascus killed at least 13 people on Tuesday, as US President Barack Obama said he would not be rushed to act on allegations Syria was using chemical weapons.
The 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.
Russia, meanwhile, banned its civilian planes from Syrian airspace after the crew of one reported coming under threat over the war-torn country.
Obama warned against rushing to judgement on Syria's use of chemical weapons 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 White House 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," he warned.
In New York, Syria's UN ambassador Bashar Jaafari alleged that an opposition group used "chemical material" during an attack near the northern city of Idlib.
Back in Damascus, state media blamed "cowardly terrorists" for a bombing that it said killed 13 people and wounded, using its term for rebels fighting to oust President Bashar al-Assad.
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.
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 flights over Syria 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" on the ground in Syria, Russian officials said without providing further details.
Interfax news agency, citing a Moscow source, said 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 on the outskirts of Mennegh airport, near Aleppo, which rebels have been trying to capture for months.
Warplanes also bombed the Jubar area of Damascus, and areas of Homs, Raqa and Latakia provinces.
Southern neighbour Jordan warned the UN Security Council of the "crushing weight" of hosting more than 500,000 Syrian war refugees.
Jordan's UN ambassador, Prince Zeid al-Hussein, said international help had been "insufficient" and that "unless the support is forthcoming then we consider this to be a threat to our future stability".
The UN has predicted there could be 1.2 million Syrian refugees in Jordan by the end of the year -- equivalent to a fifth of the kingdom's population.