Western and Arab nations vowed Thursday to step up assistance for Syria's moderate opposition, while the US said "raw data" suggested chlorine had been used as a chemical weapon in the conflict.
The Friends of Syria group meeting in London poured scorn on Syrian president Bashar al-Assad's regime's plan to hold presidential elections in June, saying it was an "insult" while the civil war was still raging.
As they met, a car bomb killed at least 29 people on the Syrian side of the border with Turkey, a monitoring group said.
US Secretary of State John Kerry said the regime's plan to hold a presidential election on June 3 was "an insult" to the Syrian people and would be a "fraud".
A joint statement from the 11 countries at the London talks described the election as "illegitimate".
The Friends of Syria group -- Britain, Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey, the United Arab Emirates and the United States -- was meeting for the first time since January.
Kerry said after the talks that "raw data" suggested chlorine had been used in Syria, supporting accusations made by France.
"I've seen the raw data that suggests that there may have been, as France has suggested, a number of instances in which chlorine has been used in the conduct of war," Kerry said.
French Foreign Minister Laurent Fabius suggested this week that the regime of President Bashar al-Assad had used chemical weapons 14 times since October, including chlorine.
- US frustration with aid blockade -
Kerry also voiced US frustration over blockages of aid to the Syrian population and said it was open to new ways of delivering essential supplies.
"We are open to the idea of providing aid through any means to get to the people who need it," Kerry said.
Under a UN-brokered agreement the Syrian regime is currently responsible for organising the distribution of aid through NGOs.
"We are very frustrated with the current process. It is not getting to the people, it is going through one gateway," Kerry said.
The UN's director of aid operations in Syria, John Ging, last week accused the government of blockading medical supplies bound for opposition areas, calling it an "abomination".
In a diplomatic boost to the Syrian opposition, Britain announced it had upgraded the status of their London office to a mission.
British Foreign Secretary William Hague said the move was being made "in recognition of the strength of our partnership" with the National Coalition headed by Ahmad Jarba.
Britain will also provide an extra £30 million ($50 million, 37 million euros) in "practical support" for the opposition, Hague said.
Jarba took part in the London talks after attending a week of high-level meetings in Washington in a bid to strengthen US support for the rebels.
In Washington, Jarba pleaded for anti-aircraft missiles to shoot down regime aircraft which are dropping deadly barrel bombs on Syrian civilians.
More blood was shed in Syria on Thursday, when a car bomb tore through a crowd at the Bab al-Salama border crossing with Turkey, the Syrian Observatory for Human Rights said.
At least 29 civilians were killed, including five women and three children.
Gruesome photographs posted online by activists showed distraught men standing over charred bodies.
A video of the scene on YouTube showed smoke rising from the twisted remains of a blown-up car.