UN inspectors will go to Syria soon to investigate three sites where chemical weapons attacks have been reported, the United Nations said Wednesday.

President Bashar al-Assad's government had blocked the inspectors since calling for a UN inquiry into the use of the banned arms in March.

"The mission will travel to Syria as soon as possible to contemporaneously investigate three of the reported incidents," said UN spokesman Martin Nesirky.

The announcement followed an accord reached with the Syrian government when two UN envoys went to Damascus last week.

The inspectors, who are led by Swedish expert Ake Sellstrom, are being assembled in Europe and could go to Syria as soon as next week, diplomats said.

The announcement that inspectors will get into Syria is seen as a major breakthrough in one of the most frightening aspects of the 28-month-old war.

The United Nations says reports on 13 different chemical attacks have been made. Syria, Britain, France, Russia and the United States have all handed over evidence to Sellstrom's team.

But experts say most of the reported attacks are now months old and there is a risk that evidence has been cleared up or has degraded.

Khan al-Assal, which is one of three sites on the initial investigation list, is now the scene of major fighting between government and rebel forces.

Syria has hundreds of tonnes of various chemical arms, according to experts, and has never joined international conventions banning their use.

The major powers all agree that chemical weapons have been used in the conflict. Russia sides with its government ally in blaming the opposition. Western nations say all the evidence points to Assad's forces using the arms in "limited" quantities.

While the initial inquiry will focus on three sites, Nesirky said UN leader Ban Ki-moon "remains mindful of other reported incidents and the mission will also continue to seek clarification from the member states concerned."

The spokesman said the first sites on the UN list includes Khan al-Assal, near Aleppo, where the government reported a chemical weapons attack on March 19. It said at least 26 people, including 16 soldiers, were killed.

Assad's government and Russia blame Syrian rebels for the attack. The Syrian opposition says Assad's forces carried it out.

The other two sites on the initial list are Ataybah near Damascus, where a suspected attack was staged in March, and Homs, where chemical weapons are alleged to have been used on December 23, diplomats said.

Britain and France have submitted evidence smuggled out of Syria to the United Nations on the Ataybah and Homs attacks, which they say were carried out by the government.

The Syrian government called for the UN inquiry in March. But it then blocked the UN inspectors, insisting that they be limited to Khan al-Assal. Ban had demanded "unfettered" access for the investigation.

Sellstrom and UN High Representative for Disarmament Affairs Angela Kane went to Damascus last week to negotiate the access accord.

Paul Walker, director of environmental security at Green Cross International, an international lobbying group, said: "It is a big step forward just to get the inspectors into Syria at all."

"Without on-site inspections there are just estimates and wide speculation and no final proof," Walker, who has monitored Syria's chemical weapons program for several years, told AFP.

He added however that the chemical residue degrades very quickly and there is a danger the attack sites have been sanitized.

"The actual on-site evidence is probably cleaned up if there was good evidence to begin with. But it will likely give the inspectors access to autopsy reports and also living victims' firsthand accounts and that could very well be helpful," he said.

Walker said Syria is believed to have up to 1,000 tonnes of chemical arms in at least a dozen stockpiles around the country.

"We assume that they are pretty secure -- there have been no reports of any weapons stolen, no reports of any rebel groups having taken over a weapons depot," he said.

He said the reported protection of the weapons has led to Western skepticism that rebel forces had used any chemical agents in the war.