Syria's army has launched a fierce assault on rebel fighters in a bid to break their year-long siege on Aleppo's central prison, according to reports on Wednesday from state media, activists and the Syrian Observatory for Human Rights.

The rebels and their allies from al-Qaeda affiliates al-Nusra Front have been trying to overrun the jail, which remains in government control, aiming to release political prisoners reportedly held in very poor conditions. A recent MEE investigation into the prison revealed that some 2,400 detainees remained in the jail where they were often denied food and medical provisions. Some 800 had already died as a result of the harsh conditions, according to the reports.

"The army, the [pro-regime] National Defence Forces militia, Arab fighters and Lebanon's Hezbollah are in fierce combat against jihadists from al-Nusra Front and Islamist rebels in the Sheikh Najjar industrial area, one kilometre from Aleppo's central prison," said the Syrian Observatory for Human Rights.

Troops loyal to President Bashar al-Assad had broken through on the road leading to the prison, said Observatory director Rami Abdel Rahman, adding that ending the siege "would constitute a strategic victory for the regime.”

State news agency SANA said, meanwhile, that "the army has taken over the village of Hilan, and is advancing towards other areas that surround Aleppo prison."

An activist in Aleppo, Mohammad Wissam, gave AFP a similar account.

"The regime wants to reach the prison. [Troops] have not managed to do so yet, but if they do...they can then reach the Castelo road, which links Aleppo's liberated [rebel-held] areas to the northern countryside," he said.

Rebels in Aleppo city, once Syria's commercial capital, rely on supply routes leading to the countryside as a rear base.

The northern countryside of Aleppo borders Turkey, a key supporter of Syria's revolt and home to hundreds of thousands of refugees fleeing the country's violence.

On Wednesday, rebels near Aleppo prison blew up al-Kindi hospital, which the loyalist army had used as a position before its takeover by the opposition.

"The building is very big, and it could be used by regime troops [should they reclaim it] to monitor supply routes used by the revolutionaries," Wissam told AFP via the Internet.

"Everyone is afraid the regime might besiege [rebel areas of] Aleppo," he added.

Meanwhile, the UN on Wednesday condemned what it called the “flagrant disregard” for international law affecting the citizens of Aleppo.

The UN High Commissioner for Human Rights criticised both the Syrian government and some armed groups for allowing the people of Aleppo to live in “terrifying conditions.”