BEIRUT -- The Syrian army inched closer to seizing a central prison in the contested northern city of Aleppo Wednesday, with intense artillery shelling and military aircraft dropping dozens of crude bombs on rebel positions in the area, Syrian state media and opposition activists said.

The sprawling prison, along with its estimated 4,000 inmates, has been caught in the deadly stalemate of Syria's civil war for months. Rebels have been besieging the facility for the past year, and have repeatedly barrelled suicide car bombs into the front gates and clashed with guards and troops holed up inside in an effort to capture the prison.

Aleppo has been carved up into rebel- and government-controlled areas since opposition fighters launched an offensive in the north in mid-2012. The Syrian army appears intent on taking opposition-held parts of the country's major cities before the presidential election on June 3.

State-run SANA news agency said army units have regained full control of the town of Heelan, near the Central Prison in Aleppo on Wednesday and are "advancing toward the surrounding areas after tightening control of the prison."

Rami Abdurrahman, the director of the Britain-based Syrian Observatory said Assad's forces are positioned about 500 metres away from the complex. He said there was heavy artillery shelling in the area, with government forces dropping at least 30 barrel bombs from military helicopters over the past 24 hours.

An activist in Aleppo who works with the Aleppo Media Center told The Associated Press that the government's push to reach the prison complex began Tuesday morning. By mid Wednesday, the "regime's tanks have come to about 500 metres away from the (prison) building," said the activist who uses the name Abu Joud al-Mujahid.

There were ferocious clashes between Assad's troops and rebels throughout Wednesday, and the opposition fighters are fast retreating from the areas because their weapons are no match for government's superior firepower.

"They are using warplanes and dropping barrel bombs from helicopters. No weapon is being spared," al-Mujahid said in an interview over Skype.

The sprawling prison lies on a highway about 4 miles (6 kilometres) north of the city of Aleppo, once Syria's prized commercial centre but now devastated by war -- with rebels controlling the eastern part of the city battling Assad's forces controlling the other part.

The rebels, a mix of Islamic groups, including the Tawheed brigade and the ultraconservative Ahrar al-Sham that is part of the Islamic Front alliance, launched their assault on the prison in April last year. Their aim was to free those inside, but also capture the government-controlled enclave amid neighbourhoods largely held by the opposition.

About 150 women are held in the prison. They detainees are a mix of common criminals, rebels and opposition activists and supporters, according to the London-based Syrian Network for Human Rights, which monitors the conflict through a network of activists on the ground. Around 1,300 of the inmates have completed their sentences but have not been freed by authorities, the group says.

At least 150 detainees have been killed in clashes around the facility that has become a symbol of a deadly stalemate of Syria's 3-year-old conflict.