"We are not able to say who has used chemical agents or chemical weapons,” said Paulo Pinheiro, chairman of the United Nations human rights investigation committee on Syria, on Friday.

Speaking to reporters after an informal meeting with UN Security Council ambassadors, Pinheiro said he would not comment on evidence, including multiple blood, tissue and soil samples, that the US, Britain and France have sent to the UN about the possible use of chemical weapons in Syria.

The technical data presented by the three countries is of limited value to the UN which, according to its rules, can pass a final judgment on the situation only after its own inspectors personally collect evidence.

Based on the unsubstantiated claim that the Syrian government has used chemical weapons against the militants, President Barack Obama ordered the CIA last week to provide arms to the anti-Syrian groups, saying the government of President Bashar al-Assad had crossed Washington’s red lines.

In an article on the Washington Post on Friday, Colum Lynch and Joby Warrick write that the US move “rests on unverifiable claims” that lack transparency.

Weapons experts say Obama’s declaration of Washington’s red line in terms of more involvement in Syria “handed the Syrian opposition a powerful incentive to fabricate evidence” against the Assad government regarding the chemical arms use, the article said.

“If you are the opposition and you hear” that the White House has drawn a red line on the use of nerve agents, then “you have an interest in giving the impression that some chemical weapons have been used,” said Rolf Ekeus, a Swedish scientist who headed UN weapons inspections in Iraq during the 1990s, the article read.