UN inspectors waiting to enter to Syria to probe mounting allegations about the use of chemical weapons will face a raft of challenges, an expert said, if and when Damascus gives the mission the go-ahead.

US President Barack Obama has warned that Syria's use of chemical weapons would be a "game changer" that crosses a "red line", but the burden of proof of attacks, allegedly carried out both by rebels and the regime, is heavy.

UN chief Ban Ki-moon on Monday issued a new plea to Syria to stop blocking a planned mission by the inspectors, saying a team now in Cyprus could be in the country within 48 hours.

President Bashar al-Assad's government has asked for a UN inquiry but has so far refused to allow in the inspection team which aims to investigate several reported incidents of chemical weapons use.

"The aim of the current investigation is not to prove that Syria has produced chemical weapons, because Syria has already implicitly admitted making them. This is about proving the weapons have been used," Sico van der Meer, an expert in chemical weapons at the Clingendael Institute in The Hague, told AFP.

"So they need to find witnesses, meaning victims if they're still alive or doctors who treated victims. They also need to carry out autopsies on bodies, but that might be difficult given that the alleged attacks happened some time ago."

Van der Meer said there could be difficulties ahead for the mission from the Hague-based Organisation for the Prohibition of Chemical Weapons if Damascus agrees to let the team in.

"Generally, this kind of team is made up of investigators who have a lot of technical knowledge, who know the specifics of chemical weapons and can detect the signs of their use," Van der Meer said.

"But also investigators who are better at the human side, who know how to find and question witnesses, who know if they're telling the truth or lying."

He said they would have to treat information from Assad's regime with caution.

"They must explore all possibilities: people are quick to point the finger at the Syrian regime, but rebels might also have found a stockpile of regime chemical weapons and use them so they can accuse the regime.

"The weapons used might also not be traditional chemical weapons such as sarin or mustard gas but also homemade chemical weapons."

Potent homemade chemical weapons could be made by mixing bleach with hydrochloric acid, to make chlorine, or bleach and ammonia to make irritant chloramines.

But even if inspectors find evidence of chemical weapons use, and of who used them, the diplomatic battle for action will only just be beginning on the world stage when their report is published.

"Syria is one of very few countries not to have signed international chemical weapons treaties, so in theory they have no legal obligation," Van der Meer said.

"The report will be a tool that will allow a diplomatic game to begin at several levels and the big question will be determining if Syria has crossed the red line.

"The international community will have to discuss things to reach agreement: has the red line been crossed and if so, what do we do? More sanctions? Military intervention?"