An Afghan policeman walks outside a prison building after an attack in Ghazni province.

AFP/Kabul

Taliban insurgents in military uniform set off a car bomb and stormed an Afghan prison on Monday, freeing hundreds of inmates and killing four policemen in the country's largest jailbreak in years.

The brazen attack in the eastern city of Ghazni comes as the Taliban ramp up attacks on government and foreign targets despite being embroiled in a bitter leadership transition.

The raid is the Taliban's third mass prison break since 2008 and marks a major blow to Afghan forces facing their first fighting season without full Nato support.

"Around 2.30 am six Taliban insurgents wearing military uniforms attacked Ghazni prison. First they detonated a car bomb in front of the gate, fired an RPG (rocket-propelled grenade) and then raided the prison," deputy provincial governor Mohammad Ali Ahmadi told AFP.

The interior ministry said 355 of the prison's 436 inmates escaped. Most were Taliban and other militants.

It added that four Afghan police officers were killed and seven wounded in the raid which left bullet-riddled bodies at the entrance of the prison.

The Taliban, who launched a countrywide summer offensive in late April, claimed responsibility.

"This successful operation was carried out at 2.00 am and continued for several hours. The jail was under Taliban control," spokesman Zabihullah Mujahid said in a statement.

"In this operation, 400 of our innocent countrymen were freed... and were taken to mujahideen-controlled areas," it added.

In the last major Afghan jailbreak in 2011 nearly 500 Taliban inmates escaped from a prison in the southern province of Kandahar.

The Taliban at the time said they sprang the inmates through a one-kilometre tunnel that took five months to dig. The government described that incident as a security "disaster".

And in 2008 about 1,000 prisoners including hundreds of Taliban rebels escaped when suicide bombers blew open the gates of Kandahar city's main prison.

Taliban insurgents are stepping up their summer offensive despite a simmering leadership succession dispute after the confirmation of the death of longtime chief Mullah Omar.

Mullah Akhtar Mansour, a trusted deputy of Omar, was named as the insurgents' new chief in late July, but the power transition has been acrimonious.

Afghan security forces, stretched on multiple fronts, are struggling to rein in the Taliban as Nato forces pull back from the frontlines.