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.
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 prime minister 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.
In July 2012, a suicide bomb attack killed Syria's defence minister and deputy defence minister and seriously wounded the interior minister.
As bloodshed continued unabated, Obama raised the issue of chemical weapons in a telephone call with his Russian counterpart Putin.
"President Obama and President Putin reviewed the situation in Syria, with President Obama underscoring concern over Syrian chemical weapons," the White House said.
Obama is under pressure because he warned last year the use or movement of chemical weapons by President Bashar al-Assad's forces would cross a US "red line".
Key political players in Washington are now saying his credibility is on the line, though the White House is seeking more detailed intelligence into exactly 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.
On Tuesday, Russia's air transport agency banned all Russian civilian flights over Syria until further notice.
The ban comes after the crew of a charter plane flying from the Egyptian resort of Sharm el-Sheikh to the Russian city of Kazan on Monday said it had come under threat when it flew over Syria.
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 source in Moscow, said unidentified assailants fired two land-to-air missiles at the plane. The aircraft did not suffer any damage and landed in Kazan on time.
The Observatory said air raids on Tuesday killed 15 people on the outskirts of Mennegh airport, near the northern city of 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.
On Monday, at least 159 people were killed in nationwide violence, said the Observatory.
Underlining the dangers of covering the conflict, Italian daily La Stampa said one of its journalists, Domenico Quirico, has not been heard from since April 9.
Media watchdog Reporters Without Borders says seven journalists are now missing in Syria, while 23 others have been killed.