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.
The Russian-backed Syrian army has captured more than a third of the 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.
The United Nations says 400,000 people are trapped inside the besieged enclave, and were already running out of food and medical supplies before the assault began with intense air strikes two weeks ago.
A senior UN official accompanying the convoy 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," Ali al-Za'tari told Reuters at the crossing point.
A World Health Organization official said the government had ordered 70% of medical supplies to be stripped out of the convoy, preventing trauma kits, surgical kits, insulin and other vital material from reaching the area.
The International Committee of the Red Cross confirmed some medical equipment had been blocked but gave no details.
The Britain-based Syrian Observatory for Human Rights said strikes targeted front lines near the town of Harasta and the villages of Beit Sawa and Hosh al-Ashari.
President Bashar al-Assad vowed on Sunday to continue the military push into eastern Ghouta, a densely populated area of farmland and towns just outside Damascus which government forces have encircled since 2013.
Many civilian residents have fled from the frontlines into the town of Douma, a resident said on Sunday.
Assad and his allies regard the rebel groups that hold eastern Ghouta as terrorists, and say a UN Security Council resolution demanding a country-wide ceasefire does not apply to operations against them.
A week ago Russia unilaterally announced five-hour daily pauses in the fighting, but clashes have continued during those hours and Western countries dismissed it as inadequate.
The fighting in eastern Ghouta follows a pattern used in other areas recaptured by the government since Russia entered the war on Assad's side in 2015, with sieges, bombardment and ground offensives combined with an offer to let civilians and fighters who surrender escape through "humanitarian corridors".
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Syrian soldiers are seen securing Syrian Arab Red Crescent trucks carrying aid at the entrance of Wafideen camp in Damascus, Syria March 5, 2018 Reuters
For the rebels fighting to oust Assad, the loss of eastern Ghouta would mark their heaviest defeat since the battle of Aleppo in late 2016 and end their ability to target the capital. Rebel shelling on Damascus has killed dozens of people during the last two weeks, state-run media has said.
The Observatory said government forces had captured a third of the area in their advance from the east. Syrian state television on Monday said the army had made major advances, seizing 40% of the area previously held by the rebels.
It broadcast live from several captured villages, showing collapsed concrete buildings, rubble-strewn streets, bullet-pocked walls and smoke rising above fields in the distance.
Late on Sunday, a rebel official said factions in eastern Ghouta were working together to prepare to retake lost ground.
The Observatory said the death toll from the offensive had exceeded 700 people in two weeks of intense bombardment.
Monday's convoy of more than 40 trucks carrying aid was the first to reach the besieged area since February 14 and only the second since the start of 2018.
Za'tari said the shipment had been scaled back from providing food for 70,000 people to providing for 27,500. The United Nations says Syria has agreed to allow in the rest of the food for the full 70,000 in a second convoy in three days.
"The convoy is not sufficient," Za'tari said. It would take many hours to offload the aid and it might be well after nightfall before it could return, he added.
The two-week assault has brought footage of children being carried out of rubble and hospitals being bombed to viewers around the world once again.
Since the fall of Aleppo more than a year ago, Syrian government forces had focussed their efforts mainly on Islamic State-held territory in the more remote east of the country, but they have now renewed their campaign to crush anti-Assad rebels in the heavily-populated west.
Moscow and Damascus deny they are killing large numbers of civilians. In comments broadcast by state television on Sunday, Assad dismissed Western statements about the humanitarian situation in eastern Ghouta as "a ridiculous lie".
At the UN Human Rights Council in Geneva, Russian diplomat Alexei Goltyaev said: "The media atmosphere is saturated with lies.... As a result we see debates and votes that are entirely removed from the actual situation on the ground."
Russia's military said the rebels who control eastern Ghouta had agreed to let civilians leave in return for aid.
Assad and Russia have both repeatedly accused the rebels of stopping civilians fleeing eastern Ghouta to use them as human shields. The insurgents deny this and say people there fear arrest, torture or forcible conscription by the government if they leave.