Beirut/Geneva: A Russian call for a five-hour truce on Tuesday failed to halt one of the most devastating campaigns of the Syrian war, where residents said government warplanes resumed striking the eastern Ghouta region after a brief lull.

Diplomatic sources meanwhile said the chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons, opened an investigation into attacks in eastern Ghouta to determine whether banned munitions were used.

Political leaders in France, the United States and Britain said this month they would back targeted military action against Damascus if there were proof chemical weapons had been used by forces under President Bashar al-Assad, Russia’s ally.

Moscow and Damascus blamed rebels for the collapse of the truce, saying fighters had shelled a safe route intended for civilians to leave the enclave. The insurgents denied such shelling, and a senior U.S. general accused Moscow acting as ‘both arsonist and firefighter’ by failing to rein in Assad.

Russian Foreign Minister Sergei Lavrov said Moscow would press on with a plan to stage similar daily pauses in the fighting, allowing aid to be delivered to eastern Ghouta through what Russia describes as a humanitarian corridor.

But the United Nations said it was proving impossible to aid civilians or evacuate wounded, and said all sides must instead abide by a 30-day truce sought by the U.N. Security Council.

‘We have reports this morning there is continuous fighting in eastern Ghouta,’ U.N. humanitarian spokesman Jens Laerke said. ‘Clearly the situation on the ground is not such that convoys can go in or medical evacuations can go out.’

Hundreds of people have died during 10 days of government bombardment of the eastern Ghouta, an area of towns and farms on the outskirts of Damascus. The assault has been among the most devastating air campaigns of a war now entering its eighth year.

The International Committee of the Red Cross (ICRC) said it was ready to access eastern Ghouta to deliver life-saving aid, but the proposed five-hour pause was too short.

Without mentioning Russia, the author of the proposal, ICRC Middle East director Robert Mardini said humanitarian corridors had to be well-planned and agreed to by all warring sides, while people should be allowed to leave of their own free will.

With its Ghouta offensive, the Syrian government is drawing on the military methods it has used to crush its opponents in other parts of Syria, including eastern Aleppo in late 2016.