* Violence threatens aid deal, only result of peace talks
* Homs governor says two vehicles reach Old Homs
* Barrel bombs in Aleppo; rebels clash in eastern Syria (Adds details on attack inside Homs city)
By Dominic Evans
BEIRUT, Feb 8 (Reuters) - An aid convoy came under fire in a besieged rebel district of the Syrian city of Homs on Saturday, threatening a United Nations-led humanitarian operation which aimed to deliver medicine and food to around 2,500 trapped people.
The Syrian Arab Red Crescent said mortar fire landed close to its convoy and shots were fired at its trucks, wounding one of its drivers. Several vehicles from the aid convoy appeared to be stranded inside the city long after darkness fell.
Syrian media said four Red Crescent workers were wounded in the incident which authorities blamed on rebels. Opposition activists accused President Bashar al-Assad's forces of staging the attack, as well as earlier mortar fire which delayed the start of the operation on Saturday morning.
The violence threatens to unravel a humanitarian deal for Homs which was the first concrete result of talks launched two weeks ago in Geneva to try to end the country's civil war.
The conflict has killed 130,000 people, driven millions from their homes and devastated whole districts of Syrian cities - particularly Homs, a centre of protest when the 2011 uprising against 40 years of Assad family rule first erupted.
At the Geneva peace talks, which resume on Monday, international mediator Lakhdar Brahimi has been pushing for agreement on aid deliveries and prisoner releases, hoping that progress on those issues could build momentum to address the far more contentious question of political transition.
But even the humanitarian talks have taken time and delivered only modest achievements, the first of which was the evacuation on Friday of 83 women, children and elderly men from the Old City of Homs. Aid workers said many showed signs of malnutrition.
Homs governor Talal al-Barazi said two vehicles carrying aid supplies entered the Old City on Saturday but that rebels had targeted the route with mortar fire, preventing any more cars from entering.
Three hours after darkness fell, several vehicles from the aid mission had not returned from rebel-held districts.
Video footage published by activists showed a convoy of eight white cars with U.N. markings and a truck stopped at a narrow street corner, already strewn with rubble.
A man with a blood on his face was led into a nearby building, before a blast struck on the far side of the truck. It was not clear how much damage was caused by the explosion.
The Syrian Observatory for Human Rights, which monitors violence in Syria, said there were reports that two Syrians had been killed and several wounded by mortar fire in Old Homs.
Syria's opposition National Coalition said on Saturday the aid operation in Homs was no substitute for lifting the siege on the remaining rebel-held area.
It said the evacuation of civilians could be "a prelude to the regime destroying the city with the remaining residents trapped inside".
"It is vital to remember that the regime has used similar tactics in the past to change the demographics of some areas in Syria," the Coalition said in a statement. "It has used similar deals to buy time to strengthen its positions on the ground and to kill more civilians."
While the aid convoy was trying to get into Homs on Saturday, fighting continued in northern and eastern Syria.
Twenty people were killed in Aleppo by barrel bombs dropped by Syrian army helicopters, the Britain-based Observatory said.
The improvised explosives, often rolled out of the cargo holds of aircraft, cause widespread and indiscriminate damage.
Hundreds of people have been killed in such attacks in Aleppo city this year and many thousands have fled rebel-held districts, seeking shelter in government-controlled neighbourhoods or trying to cross the Turkish border.
The air offensive has also helped Assad's forces take back some ground in Syria's biggest city, which has been contested since the summer of 2012 when rebel forces swept in from Aleppo's rural hinterland to take over around half the city.
Since then the army, backed by Iranian military commanders, Lebanon's Hezbollah militia and Iraqi Shi'ite fighters, has taken back territory around Damascus and Homs.
Infighting between rival rebel forces, including foreign Sunni Muslim jihadis and al Qaeda-linked fighters, has also helped Assad's counter-offensive.
Several Islamist groups have been fighting the Islamic State in Iraq and the Levant (ISIL), an al Qaeda splinter group, across northern and eastern Syria for several weeks.
On Saturday the Britain-based Observatory reported heavy fighting in the eastern province of Deir al-Zor after the al Qaeda-linked Nusra Front and another Islamist group, Ahrar al-Sham, attacked ISIL, accusing it of seizing control of oil fields and other key installations.
It said at least 20 people were killed in fierce clashes in Deir al-Zor city and elsewhere in the province, which borders the Iraqi province of Anbar where militants including ISIL fighters overran two cities last month.
Syria's uprising turned into an armed insurgency after demonstrations were put down with force and has now degenerated into a civil war pitting regional Sunni and Shi'ite powers against each other and destabilising the wider Middle East. (Additional reporting by Stephanie Nebehay in Geneva; Editing by Ralph Boulton)