By Syed Shoaib Hasan

BBC News, Islamabad



Robbers have stolen at least $3.7m from a bank in Karachi, in what is being called Pakistan's biggest bank heist.

The robbery took place at the main branch of Allied Bank, one of Pakistan's largest banks, less than 100 metres from city police headquarters.

Police say the gang was led by one of the bank's guards. The robbers broke into the main vault and stole foreign currency worth 311m rupees ($3.7m).

Police have launched an investigation, but have so far made no arrests.

'Tied staff up'

The Allied bank branch is located on Karachi's II Chundrigar Road, which houses the Karachi Stock Exchange and head offices of trading and financial firms operating in Pakistan.

Police believe the robbers' leader worked for a private security agency and was employed at the branch using a fake identity.

Officials say he arrived early for his shift on Monday and was joined minutes later by his accomplices.

"The men tied up the few staff present at the time, and then used acetylene gas cutters to break into the main vault," police said.

The gang then made off with dollars, euros and pounds stored inside the vault.

There were about a dozen bank robberies in Karachi in 2009, although none approached this one for sophistication and planning.

Most were hit-and-run raids on armoured cars or foreign currency exchange companies.

The last bank robbery in Pakistan in which more than 100m rupees was stolen was in 2007 when two security guards stole 160m rupees at gunpoint from a foreign currency exchange company in Karachi.

Several recent bank robberies in Pakistan's financial capital have been traced to the Taliban.

Police will be looking for any connection between the robbers in this latest case and militants active in Pakistan and Afghanistan.