Aid trucks reached Syria's eastern Ghouta region on Monday for the first time since the start of one of the war's deadliest assaults, but the government stripped some medical supplies from the convoy and pressed on with its air and ground assault.

A World Health Organization official said the government had ordered 70 per cent of medical supplies to be stripped out of the convoy, preventing trauma kits, surgical kits, insulin and other vital material from reaching the besieged, rebel-held area.

The International Committee of the Red Cross confirmed some medical equipment had been blocked but gave no details.

Members of the Syrian Red Crescent distribute supplies to civilians in Douma, in eastern Ghouta on Monday. (Syrian Red Crescent via AP)

Photographs posted on Syrian Red Crescent's Twitter account showed aid trucks distributing aid in Douma in eastern Ghouta.

An international aid convoy crossed a final Syrian army checkpoint and began to enter the besieged, rebel-held enclave of eastern Ghouta, a Reuters witness said on Monday.

Ali al-Za'tari, a senior United Nations official accompanying the convoy, said it would take "many hours" to offload the aid in the enclave and that it might be "well after nightfall" before it could leave eastern Ghouta.

Abu Mohammad Alaya, 50, is helped out from where he was sheltering in Douma in eastern Ghouta on Feb. 22. (Bassam Khabieh/Reuters)

Al-Za'tari said the convoy consists of 46 truckloads of supplies, including food for 27,500 people.

He said he was "not happy" to hear loud shelling near the crossing point into eastern Ghouta despite an agreement that the aid would be delivered in safety.

"We need to be assured that we will be able to deliver the humanitarian assistance under good conditions," al-Za'tari told Reuters at the crossing point.

The only other UN delivery of aid to eastern Ghouta this year was on Feb. 14, when a convoy with assistance for 7,200 people reached Nashabiyah, one of three eastern Ghouta towns regime forces seized control of last Saturday.

Pawel Krzysiek of the International Committee of the Red Cross said Monday's convoy had arrived in Wafideen, a key crossing point set up by the Syrian government for civilians wishing to leave eastern Ghouta and also for aid to enter the enclave.

"Feels like racing with time," Krzysiek said in a tweet.

Eastern Ghouta, home to some 400,000 people, has been under a crippling siege and daily bombardment for months. Hundreds of civilians have been killed in the last two weeks alone.

The Britain-based Syrian Observatory for Human Rights said the latest airstrikes targeted front lines near the town of Harasta and the villages of Beit Sawa and Hosh al-Ashari. The Observatory said 45 people were killed and 190 injured on Monday, bringing the death toll from the offensive to more than 740 people.

The Russian-backed Syrian army has captured more than one-third of eastern Ghouta in recent days, threatening to slice the last major rebel-held area near the capital in two, despite Western accusations of violating a ceasefire.

Late on Sunday, a rebel official said factions in eastern Ghouta were working together to prepare to retake lost ground.