* Convoy delivers aid to Ghouta amid shelling
* Army retakes most farmland in onslaught, eyes towns
* Rebel group allows imprisoned Nusra fighters to leave enclave
* Trapped residents in dilemma - seek aid or stay inside (Updates with evacuation of Nusra Front prisoners, edits)
BEIRUT, March 9 (Reuters) - A convoy of relief trucks crossed front lines into the rebel-held Damascus suburb of eastern Ghouta on Friday and unloaded all its food to the trapped population, despite fighting that the Red Cross said had come "extremely close".
The Syrian government's two-week onslaught in eastern Ghouta has killed around 950 civilians, the war monitoring group the Syrian Observatory for Human Rights said.
For eastern Ghouta's civilians, mostly trapped underground with little food or water, the choice of whether or not to emerge to seek supplies may be one of life or death.
"People were hopeful after the bombardment decreased and went out onto the streets," said Moayad al-Hafi, a man in the Ghouta town of Saqba. "But then air strikes began again, and there are still people under the rubble that we couldn't get out."
Syria and its main ally Moscow have both said the assault is needed to stop rebel shelling of nearby Damascus and end the rule of Islamist insurgents over civilians in eastern Ghouta, where some 400,000 people live.
But U.N. human rights chief Zeid Ra'ad al-Hussein has said, in comments criticised by Syria's government, that the assault is "legally, and morally, unsustainable".
There was a pause in the government's bombardment overnight, but air strikes and shelling quickly resumed after the convoy carrying food parcels crossed into eastern Ghouta, according to residents and the Observatory.
Syrian state television and a witness later said bullets and mortars were fired from inside the rebel enclave at the al-Wafideen crossing point, through which the convoy had entered.
"Shelling in proximity of Douma (in) eastern Ghouta today is putting the ... convoy at risk," said U.N. resident coordinator Ali al-Za'tari said.
He said the fighting had resumed "despite assurances of safety from parties including the Russian Federation".
Robert Mardini, Middle East director of the International Committee of the Red Cross, said it had been "taken aback by the fighting that broke out despite guarantees."
A Douma resident, in a voice message over which loud explosions were audible, said four jets were in the sky and residential areas had come under air attack.
The food was supposed to be delivered on Monday when a previous convoy entered Douma, but fighting forced it to leave early without unloading fully.
The 2,400 food parcels that were delivered can sustain 12,000 people for one month, and 3,248 bags of wheat flour were also unloaded, according to the ICRC.
For President Bashar al-Assad, the capture of Ghouta would mark a major victory as he builds on the military momentum created by Russia's 2015 entry into the war, which has restored his rule over swathes of Syria.
In around two weeks, the Syrian army has retaken nearly all the farmland in the enclave east of Damascus under cover of near-ceaseless shelling and air strikes, leaving only a dense sprawl of towns - about half the territory - still under insurgent control.
Around a dozen imprisoned fighters from a group linked to al-Qaeda's former affiliate in Syria were allowed to leave the besieged enclave on Friday.
One of the area's two main rebel groups, Jaish al-Islam, said it had agreed with the United Nations to allow the evacuation of fighters that it was holding from Tahrir al-Sham, an alliance of jihadist factions linked to the Nusra Front.
Jaish al-Islam and the other main rebel faction, Failaq al-Rahman, have repeatedly denied that they are in negotiations with Damascus or its ally Russia over their own evacuation.
In other areas such as Aleppo, rebels have eventually surrendered terrain in return for safe passage to other opposition areas along with relatives and civilians loath to fall back under Syrian President Bashar al-Assad's rule.
In a statement, Jaish al-Islam said the imprisoned fighters would be sent to a rebel-held area in northwest Syria.
Syrian state television then showed footage that it said was of 13 fighters and their families leaving the enclave.
Air strikes started up again after the evacuation, killing nine people and injuring around 28, the Observatory said.
"ONE MEAL IN SEVERAL DAYS"
U.N. aid agencies have pleaded with the Syrian government and its ally Russia to halt the campaign and let aid in.
Bilal Abu Salah, a resident of Douma, said shortages were causing great hardship. "Entire families eat one meal in several days," he said.
The Syrian government has opened what it calls safe routes out of the enclave, but no civilians are known to have left yet.
Damascus and Moscow accuse the insurgents of shooting at civilians to prevent them fleeing. A Reuters reporter said there was small arms and mortar fire from rebel areas on the al-Wafideen crossing on Friday.
Rebels deny stopping anyone leaving and say people have refused to cross into government territory for fear of persecution.
However, the terror of the bombardment and the desperate living conditions are taking their toll and may push people to brave the fighting and flee.
"I don't want to leave, but I don't want any harm to happen to my family," said Douma resident Abu Ahmad al-Ghoutani, who said he has two children.
State media have reported people in eastern Ghouta raising Syrian government flags and holding small protests in support of Assad. The Observatory has reported protests in one village to demand an end to the bombardment and the departure of rebels.
Tens of thousands of people have fled further into the enclave in the face of the warfare, a U.N. official said on Thursday, and residents of Douma said shelters were crowded with the new arrivals. (Reporting by Ellen Francis, Lisa Barrington, Dahlia Nehme and Yara Abi Nader in Beirut, Kinda Makieh in Damascus and Stephanie Nebehay in Geneva; writing by Angus McDowall; Editing by Kevin Liffey)
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