First aid convoys reach Syria's Ghouta

A WHO official says Syrian troops have stripped most medical material from UN vehicles, preventing surgical kits from reaching 400,000 people in eastern Ghouta.

Civilians remain trapped in eastern Ghouta as the Syrian government forces continue their operation. (AAP)

Aid convoys have reached a crossing point into Syria's eastern Ghouta, a witness says, bringing the first relief to the besieged enclave since one of the deadliest government assaults of the seven-year war began two weeks ago.

A World Health Organization (WHO) official said government authorities had stripped most medical material from UN vehicles, preventing surgical kits, insulin, dialysis equipment and other supplies from reaching the enclave of 400,000 people.

President Bashar al-Assad vowed on Sunday to continue a military push into the biggest remaining opposition stronghold near Damascus, saying the offensive did not contradict five-hour ceasefires arranged each day by his main ally Russia.

A wider, full ceasefire backed by the UN Security Council has not come into effect during the campaign, which began with massive government air strikes two weeks ago and has continued in recent days with a ground assault.

The Syrian Observatory for Human Rights, a Britain-based war monitor, said the death toll from the offensive had exceeded 700 people in two weeks of intense bombardment on the densely populated region of farmland and towns.

Pro-Assad forces have made sudden advances into eastern Ghouta in recent days, capturing a third of the area, the Observatory said, and bringing them close to cutting it in two, in what a rebel official called a "scorched earth" attack.