LONDON // Western and Arab countries promised on Thursday to step up assistance for Syria’s moderate opposition, while the US said it had information that chlorine had been used as a weapon in the conflict.

Diplomats attending the Friends of Syria group meeting in London also poured scorn on the Syrian regime’s plan to hold presidential elections in June, saying it was an “insult” while the civil war continued.

As the group met, a car bomb killed at least 29 people on the Syrian side of the border with Turkey.

John Kerry, the US secretary of state, who earlier met foreign ministers from Gulf Arab countries, 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, the UAE, Britain, Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey, and the United States, met for the first time since January.

Mr 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,” Mr Kerry said.

The 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.

Mr 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.

Mr Kerry said: “Every possible avenue that is available will be pursued by one country or another. We are open to the idea of providing aid through any means that will get to people who need it and while a decision has not categorically made, we are open to anything.”

But he declined to say whether that would include arming the Syrian rebels.

NGOs complain that despite multiple requests, the United Nations has so far failed to share its methodology in identifying those most in need and monitoring where its aid goes after delivery. Often it does not even disclose what its food aid includes.

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.

The move was made “in recognition of the strength of our partnership” with the National Coalition headed by Ahmad Jarba, said the British foreign secretary, William Hague.

Britain will also provide an extra £30 million (Dh185m) in “practical support” for the opposition.

Mr 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, Mr Jarba pleaded for anti-aircraft missiles to shoot down regime aircraft which are dropping deadly barrel bombs on Syrian civilians.

Human Rights Watch claimed this week that the regime was placing cylinders of chlorine gas in some of its barrel bombs.

Heavy fighting continued 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.

Car bombings have become common in Syria as the influence of Islamist extremist groups has risen. Opposition activists have blamed Al Qaeda-linked fighters, who are engaged in fighting between rival rebel factions in Syria, although no group claimed responsibility for Thursday’s blast.

An amateur video posted online showed women, men and children at the scene of the blast in the northern province of Aleppo. Several cars and motorcycles were ablaze.

Rebels captured the border crossing on the Syrian side in July 2012, opening a crucial transit point for people and supplies. But the area has seen an uptick in clashes and attacks between rebel groups fighting for control of the crossing in recent months.

“Oh God, may you punish them!” a man said as people used fire extinguishers to put out flames consuming two vehicles.

* Agence France-Presse with additional reporting by Reuters and Associated Press