By Anthony Deutsch and Peter Apps
AMSTERDAM/LONDON (Reuters) - The longer chemical weapons inspectors wait in a Damascus luxury hotel for permission to drive up the road to the site of what appears to be the worst poison gas attack in a quarter century, the less likely they will be able to get to the bottom of it.
The poisoning deaths of many hundreds of people took place only three days after a team of U.N. chemical weapons experts arrived in Syria. But their limited mandate means the inspectors have so far been powerless to go to the scene, a short drive from where they are staying.
"We're being exterminated with poison gas while they drink their coffee and sit inside their hotels," said Bara Abdelrahman, an activist in one of the Damascus suburbs where rebels say government rockets brought the poison gas that killed hundreds of people before dawn on Wednesday.
The Syrian government denies it was behind the mass killing, the deadliest incident of any kind in Syria's two-and-a-half year civil war and the worst apparent chemical weapons attack since Saddam Hussein gassed thousands of Iraqi Kurds in 1988.
The United Nations has asked President Bashar al-Assad's government for access to the scene, and Secretary-General Ban Ki-moon said it should be investigated "without delay".
Former weapons investigators say every hour matters.
"The longer it takes, the easier it is for anybody who has used it to try to cover up," said Demetrius Perricos, who headed the U.N.'s team of weapons inspectors in Iraq in the 2000s.
"The more you cover up, the more time it takes afterwards to uncover it. So time is definitely not something that you want to take, you don't want to do it slowly," Perricos told Reuters.
Chemical weapons experts say there is little doubt that it was exposure to poison gas of some kind that killed the hundreds of victims, although exactly what chemicals were used could not be determined from just looking at images.
"Clearly, something has killed a lot of people," says Dan Kaszeta, a former U.S. Army chemical officer and Department of Homeland Security expert now a private consultant. "We're not going to know what until someone gets a sample."
Stephen Johnson, a former British Army officer specializing in chemical, biological and nuclear warfare and now visiting fellow at Cranfield University's forensic unit, said it was also "staggeringly effective if it is a chemical attack, which implies more than a casual rocket or two."
Keeping U.N. inspectors from reaching the site would not stop Western countries from obtaining their own evidence and drawing their own conclusions, as in previous cases when they determined Assad's forces used smaller amounts of sarin gas.
In the past, France, Britain and the United States obtained samples of soil and human tissue and other evidence they said proved Assad's government was to blame.
But unless the U.N. team obtains its own evidence, it could be difficult to build an international diplomatic case. Proving beyond reasonable doubt who was responsible might require evidence such as ballistic analysis that shows where any missiles came from.
The incident exposes the limits of the mandate of the U.N. team led by Swedish scientist Ake Sellstrom, details of which have not been fully made public.
Movements of the 20-member team are limited to locations previously agreed with Assad's government. The team is authorized to investigate only three alleged incidents, at least one of which is a case in which the government says chemicals were used by rebels, and the other two of which have not been disclosed.
Western diplomats say they had hoped that once on the ground the team would be able to broaden its mandate.
"They probably have their hands tied as to where they can go because someone has territorial control," said Hans Blix, who headed a team of weapons inspectors in Iraq. "I can well imagine that objections will be raised on the ground that the security of inspectors cannot be protected in the area in question."
Rebels said on Wednesday they would provide protection for inspectors at the site.
Rolf Ekeus, a Swedish diplomat who worked with Sellstrom in Iraq, said Sellstrom had the expertise needed to determine what happened, if he were given access.
"Sellstrom is extremely well familiar with all these capabilities, that is his specialty," Ekeus told Reuters.
France has led international calls for force to be used if it is proved that the government is behind the attack. Rebels and opposition activists say the diplomatic wrangling over the mandate of the inspectors provides cover for countries that lack the will to act.
A year ago, U.S. President Barack Obama announced that using chemical weapons would be a "red line" that would prompt serious international action. Three months ago, he concluded Syria had crossed the line by using sarin, and said Washington would respond by providing military aid to the rebels, but U.S. hardware has yet to arrive.
"We are all very frustrated by what has happened and the lack of action," Qassim Saadedine, a commander and spokesman for the rebels' Supreme Military Command told Reuters on Thursday.
"As usual, there is a lot of talk and outrage but for the people suffering on the ground it is hard to take that seriously. After all this time, these statements are empty words to us."
(Additional reporting by Fredrik Dahl in Vienna and Erika Solomon in Beirut; Editing by Peter Graff)