* Evacuations had been delayed for 48 hours
* Rebels say prisoners to be freed as part of deal (Adds first released prisoners arriving in rebel-held area)
By Ellen Francis
BEIRUT, April 21 (Reuters) - The evacuation of Syrian civilians and fighters from four besieged towns was completed on Friday after a 48-hour halt, as part of a mediated swap deal between the warring sides, state media and a rebel official said.
Thousands of civilians and pro-government fighters from the Shi'ite towns of al-Foua and Kefraya arrived in army-held Aleppo, a war monitor said.
Evacuees from the two rebel-besieged towns had been stuck at a staging area outside the city, where a bomb attack on an evacuation convoy killed scores of people last week.
In exchange, busloads of rebels and their relatives from Zabadani left a second nearby transit point for rebel territory, state television said. A witness told Reuters that 100 prisoners released by the government as part of the swap had also arrived in the rebel-held territory outside Aleppo.
The towns of Zabadani and Madaya, which had long been under siege by pro-government forces near Damascus, came under state rule this week after Sunni rebels and civilians were evacuated.
The Syrian Observatory for Human Rights, a Britain-based war monitor, said the 48-hour suspension was due to rebel demands for the government to free 750 prisoners under the deal.
Rebels reached an agreement on Friday with the government for it to release the first 500 prisoners who would cross into opposition areas as part of the wider deal, a rebel official told Reuters. The other 250 prisoners would be released in coming days, he said.
Mohamad Abu Zeid, a spokesman for the Ahrar al-Sham group, said negotiations over the issue had concluded and the prisoners would arrive at a rebel-held area near Aleppo "within hours".
The Syrian army could not immediately be reached for comment.
A senior Arab diplomat in Qatar involved in the talks told Reuters that Iranian officials and Ahrar al-Sham held discussions about the four-towns swap in Qatar when Iran's foreign minister visited Doha in March.
Those discussions also involved the freeing of 26 Qatari hostages held by unidentified gunmen in Iraq, he said.
During evacuations on Saturday, a bomb blast hit a packed convoy carrying evacuees from al-Foua and Kefraya, killing at least 126 people, including more than 60 children, who were waiting on Aleppo's outskirts.
Syrian President Bashar al-Assad was cited as saying the former Nusra Front carried out the bombing, which no group has yet claimed responsibility for.
The Nusra Front was al Qaeda's affiliate in Syria until it broke off its allegiance to the global militant movement and rebranded itself last year. It merged with a number of insurgent Islamist factions in January under a new jihadist alliance called Tahrir al-Sham.
Thousands of Syrians have been evacuated mostly out of besieged opposition areas in recent months, under deals between Assad's government and rebels fighting for six years to unseat him.
The opposition has denounced this as a deliberate policy of demographic change to forcibly displace Assad's opponents away from the main cities of western Syria.
Fighters and civilians have poured into rebel-held Idlib province in northwest Syria over the last year, shuttled out of other areas the army and its allies captured.
With the help of Russia and Iranian-backed militias, the government has gained the military upper hand against the wide array of rebel groups, including some supported by Turkey, the United States and Gulf monarchies.
U.N. Syria humanitarian adviser Jan Egeland said there had been more evacuation deals this year than before, but that they appeared driven more by military priorities than humanitarian concerns.
"They seem to follow a military logic, they do not seem to put the civilians at the heart of the agreement," he told reporters in Geneva on Thursday.
The United Nations was not involved in the evacuation of the four towns. Egeland said that it was misleading to consider them voluntary evacuations when the towns had been besieged for years.
"Besiegement should end by being lifted," he said, "not by places being emptied from people." (Additional reporting by Angus McDowall in Beirut, Stephanie Nebehay in Geneva, Tom Finn in Doha; Editing by Alison Williams)