AMSTERDAM: As UN weapons inspectors came under fire in Syria on Monday, the evidence of an apparent large-scale chemical weapons attack they are seeking is already fading from the scene.

The longer it takes the 20 experts to get to the spot where rockets carrying nerve agents are said to have killed hundreds of people on Aug. 21, the harder it will be for the mission led by Swedish scientist Ake Sellstrom to find meaningful remnants of chemical weapons.

Traces of chemicals on munitions fragments, buildings and impact craters will already have degraded. It will also have become difficult to detect anything in the urine of inhabitants in the outskirts of Damascus. Perpetrators will have had days to try to cover up proof of the attack, experts said.

Ralf Trapp, a disarmament expert who worked for the Organization for the Prohibition of Chemical Weapons, which is supplying experts to the UN team, said traces of chemicals in a victim’s urine fade within days, though blood could contain traces for weeks.

“They should be collected as soon after the incident as possible, preferably within a couple of weeks after the alleged use,” Trapp said.

In a conflict that is dividing world powers, inspectors will also have to safeguard the integrity of the samples.

They have to make sure containers and vials transported to the laboratories for analysis follow a strict chain of custody, with fiber-optic seals and accompanied by exhaustive documentation “to be able to demonstrate that the samples have not been tempered with,” Trapp said.

Samples should be sent to two or preferably three designated laboratories from among those in the 20 countries with which the OPCW has working agreements.

Jean Pascal Zanders, an independent expert who runs chemical weapons blog www.the-trench.org, said other sources of evidence, such as witness interviews, could also be used to support claims of a large-scale chemical attack.

Autopsies are another option, he said, but under Muslim tradition, the dead are buried within 24 hours, and families would have to grant permission for the remains of their relatives to be exhumed.

Iraq meanwhile said Monday it opposes the use of its airspace or territory in any attack on Syria.

“We do not agree to any use of our airspace ... to attack any neighboring country through our land,” Prime Minister Nuri Al-Maliki’s spokesman told AFP, asked if Baghdad would authorize the use of its airspace in any action against Assad regime.

Germany on Monday said it would support “consequences” against the Syrian regime if its use of chemical weapons is confirmed.

“The use of chemical weapons of mass destruction would be a crime against civilization,” Foreign Minister Guido Westerwelle told a Berlin conference of German ambassadors.

“If their use is confirmed, the world community would have to act. Then Germany would be among those who would support consequences.

Chancellor Angela Merkel’s spokesman said there was “a very high probability” the regime had employed chemical weapons, which he said would constitute “a terrible crime against men, women and children.”

British Prime Minister David Cameron is cutting short his holiday to deal with the Syria situation.

Cameron, who has been on holiday in Cornwall, southwest England, held a series of phone conversations with other world leaders over the weekend but will be returning to London to continue working on the international response.