The Syrian government has warned that rebels may acquire chemical weapons from foreign sympathisers to use against civilians and then blame the Assad administration.

"What raises concerns regarding this news circulated by the media is our serious fear that some of the countries backing terrorism and terrorists might provide the armed terrorist groups with chemical weapons," the Syrian foreign ministry said in a letter to the UN, according to a Xinhua report.

Those countries may "claim that it was the Syrian government that used the weapons".

The letter blames the international community for its lack of action over recent developments such as rebels taking control of a chlorine processing plant near the northern city of Aleppo.

Earlier, Syrian rebels declared Damascus international airport a legitimate target and reportedly cut access to it from most sides.

'Enough Evidence' on Syria's Chemical Weapons

The Syrian government warning came in response to reports from the US and the UK that there was evidence that the Syrian government was preparing to use chemical weapons against its own people.

British Foreign Secretary William Hague said that he had intelligence that the Assad regime was preparing to use chemical and biological weapons against the rebels in a desperate attempt to win over the opposition.

"We have seen some evidence of that. We and the US, as I said in parliament this week, have seen some evidence of that and that is why we have issued strong warnings about it. We have done so directly to the Syrian regime," Hague told the BBC ahead of a 'Friends of Syria' conference in Morocco on 12 December.

Though Hague did not divulge the nature of his information, he said he had seen "enough evidence".

"We absolutely cannot be specific about that because clearly those are intelligence sources that these things come from. But we have seen enough evidence to know that they need a warning and they have received that warning."

Earlier, US President Barack Obama warned the Assad regime of "consequences" if the administration used chemical weapons against Syrians. In a letter to the Syrian leader, UN Secretary-General Ban Ki-moon urged Assad "to refrain from the use of any such weapons under any circumstances".

Rebels Form New Military Command

Meanwhile, Syrian rebel commanders have elected a 30-member unified command in a bid to unite the disparate groups fighting Assad forces in Syria.

The new body - the Supreme Military Council, with chief of staff Brigadier Selim Idris, a former Syrian army officer - was elected by a gathering of 500 delegates in Turkey. The new command group is seen as a united armed front against the Assad regime.

''The aim of this meeting was to unify the armed opposition to bring down the regime. It also aims to get the situation under control once the regime falls,'' a rebel commander who attended the meeting told the Associated Press.

The unified command has excluded extremist groups such as Jabhat al-Nusra, which is thought to be linked to al-Qaida and another fundamentalist group Ahrar al-Sham, a move designed to ensure the continued support of Western countries.

In November, the Syrian opposition political leadership formed a new National Alliance as an alternative to the Assad government. Several nations, including Britain and France, have recognised the political grouping as Syria's "government in exile". The US is also expected to recognise it at the forthcoming Friends of Syria conference.