MOSCOW (AFP) - Russia on Sunday welcomed Damascus' offer to allow a mission by UN inspectors probing alleged chemical weapons use and warned the West that military action against the Syrian regime would be a "tragic mistake".

"We strongly urge those who, by attempting to impose their own results on the UN experts, are raising the possibility of a military operation in Syria to use their common sense and refrain from committing a tragic mistake," foreign ministry spokesman Alexander Lukashevich said in a statement.

The United Nations said its inspectors could begin work as early as Monday, after Damascus agreed to the probe on Sunday during a visit by top UN official Angela Kane.

Washington, which has said that evidence President Bashar al-Assad's regime used chemical weapons would cross a red line and warrant tougher action, said the Syrian offer was "too late to be credible".

Russia - which together with Iran and China has supported Assad throughout the 29-month-old Syrian crisis - urged the rebel camp to guarantee the safety of the UN team lead by professor Aake Sellstroem.

"It is important that the armed opposition controlling part of Eastern Ghouta ensure that the UN mission is able to operate safely and refrain from the kind of armed provocation witnessed against UN observers last summer," he said.

Opposition leaders already said Friday that UN inspectors would have "unfettered" access to areas under rebel control such as Eastern Ghouta on the outskirts of Damascus to investigate the use of chemical weapons.

Doctors Without Borders has said 355 people died last week of "neurotoxic" symptoms, after the opposition claimed regime forces unleashed chemicals east and southwest of Damascus last Wednesday causing more than 1,300 deaths.

The regime has denied the charges and in turn accused the rebels of using chemical arms.