Men are seen walking near damaged buildings in the besieged town of Douma, Eastern Ghouta, Damascus, Syria. Picture: FAROOQ KHAN/EPA-EFE

Beirut - The bombardment of Syria's rebel-held Eastern Ghouta enclave outside Damascus killed at least 46 people and wounded about 190 others on Monday, the Syrian Observatory for Human Rights said.

The enclave is shrinking in the face of a Syrian government offensive, ongoing despite calls for a nationwide ceasefire to the long-running civil war.

A siege of the enclave, where an estimated 400 000 people live, has been in place since 2013.

Activists and world leaders warn the situation is dire, with food and medicine running short as the army intensifies its airstrikes.

Earlier on Monday, an aid convoy of 46 trucks entered the enclave.

The convoy carried food for 27,500 people as well as medical and surgical items, according to Ingy Sedki, a spokeswoman for International Committee of the Red Cross (ICRC) in Syria.

A Russia-brokered ceasefire has been in place for five hours daily since late last month.

Read: Stripped of medical supplies, first aid convoy reaches Syria's Ghouta

"The convoy is a positive first step and will lessen the immediate suffering of some civilians in the Eastern Ghouta region," said ICRC's Middle East Director Robert Mardini.

"But one convoy, however big, will never be enough given the dire conditions and shortages people are facing. Repeated and continuous humanitarian access is essential and more must be granted in the coming period," he added.

Despite the progress made Monday, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said the Syrian government blocked some critical health supplies as part of its aid shipment.

"The items included trauma kits and other life-saving supplies," the agency said. "Consequently, three of the 46 trucks being sent to [Eastern Ghouta] today are close to empty."

The only UN delivery of assistance to Eastern Ghouta this year was nearly three weeks ago, when a convoy carrying assistance for 7,200 people arrived.

OCHA wants to deliver enough aid for 70,000 people in Eastern Ghouta and said it has received assurances that more trucks can be moved in on Thursday.

According to the Britain-based Observatory, Syrian government and allied forces pushed deeper into Eastern Ghouta on Monday and are now in control of 35 per cent of the rebel-held enclave.

"The regime is trying to divide Eastern Ghouta into two sectors north and south and only 3 kilometres remain for the regime to achieve this in the enclave," Rami Abdel Rahman, head of the Observatory, told dpa.

They are mainly focusing their attacks on the east and south-eastern part of Eastern Ghouta, he said.

He added that hundreds of people were displaced inside Eastern Ghouta over the weekend.

"They are mainly moving towards the centre of the enclave," he said.

Abdel Rahman said there were reports of heavy shelling in southern and northern Ghouta as the convoy reached the enclave.

Continued violence has marred the truce declared by Russia, a major ally of Syrian President Bashar al-Assad.

In Geneva, the UN human rights body launched an investigation into the recent Syrian army attacks on civilians trapped in Eastern Ghouta, in a resolution passed Monday.

The Geneva-based body strongly condemned the bombardment of hospitals as war crimes, and it also denounced the blockade of aid convoys and the alleged use of chemical weapons in the area, which was described by Secretary General Antonio Guterres as "hell on earth".

The Human Rights Council urged all sides in the conflict to implement a month-long ceasefire across Syria that was requested by the UN Security Council.

The United States has accused Russia of not sticking to that agreement, passed unanimously on February 25, a charge that Moscow denies.