Image caption Karachi's police travel under tight security for fear of attack

Police in Pakistan's largest city, the port of Karachi, say the Taliban are increasing in numbers and appointing new leaders.

The sprawling southern city, with its population of about 18 million, is an ideal place for militants to get lost in the crowd.

"The Taliban are clever enough to keep a low profile, but their presence is growing," said a local police chief who has been fighting them for years.

He is in charge of Sohrab Goth, a down-at-heel district of four million on the edge of the city, which is a Taliban safe haven. The chief asked not to be named.

"I'm on enough hit lists already," he said.

Karachi has been under an intense international spotlight since the failed Times Square attack.

Image caption Sohrab Goth's narrow streets make it an ideal hiding place

Faisal Shahzad, the Pakistani-born US citizen who has pleaded guilty to carrying out the attack, had Karachi connections.

During a trip there last year he is alleged to have met a militant go-between. Shahzad has been charged with terrorism offences in an indictment which alleges he received training and financial help from the Pakistan Taliban.

They are consolidating their operations in Karachi, according to the police chief. He says they are creating a new tier of local leaders in areas they control, rather than relying on one overall 'Emir', as in the past.

Pakistan's financial capital offers the Taliban rich pickings. For the most part this is where they make money, not where they strike.

They raise funds through extortion, bank robberies and kidnappings - money that is funnelled back to training camps and bases in the tribal areas, where suicide bombings are planned.

The chief took us on a tour of the dust-caked streets of Sohrab Goth in his one-and-only armoured personnel carrier. It's so new the plastic is still on the seats.

We visited an industrial area where the Taliban operated a simple but effective extortion racket by padlocking factories' doors. Business owners had to pay the militants to remove the locks.

Police say when they moved in on the gang involved last June, a gun battle erupted. Five militants were killed on the spot, but a few managed to melt away.

That's easy to do here.

We were shown a warren of narrow alleyways where they come to take cover, and to stash their spoils. The streets are so narrow, police cannot drive in.

"We can't patrol," the chief said. "They can commit crimes in any part of Karachi and come back here to hide. It's very difficult to make arrests here."

Sohrab Goth has another great advantage for the militants. They blend in, among their own people. The locals are Pashtuns, the same ethnic group as the Taliban.

Western recruits

But the militants are eager to broaden their horizons beyond Pakistan, according to a former foot soldier.

The 21-year-old, who is now lying low in Karachi, told us they were looking to the West for recruits. He said the issue was discussed in Waziristan, where he was trained.

"The discussion at meetings was that we should try to meet people from Britain or the United States or other countries and we should tell them it's their duty to recruit people," he said.

Image caption The Taliban want recruits from the West, a former militant tells the BBC

The former fighter said he was recruited at the age of 17 and fought with the Taliban in the Swat valley before parting company with them last year, at his mother's request.

He answered our questions without hesitation but kept the meeting brief, due to his fear of arrest.

Security experts say the Taliban in Karachi present a new threat for Pakistan and the West, because they are forging links with other militant networks and splinter groups.

"They have started to co-operate," said another senior police officer, who also spoke anonymously.

"They can help each other, and these splinter groups can do anything."

One study of militant strength in the city suggested as many as 17 different militant organisations and splinter groups were present.

Groups are getting harder to track because they are fracturing, according to Muhammed Amir Rana of the Pakistan Institute for Peace Studies, who monitors militant networks.

"Authorities will only know about new groups after they have committed an attack," he said.

While the Times Square attack failed, Faisal Shahzad managed to pass unnoticed from the shadowy back streets of Karachi to the bright lights of Times Square. The question for investigators in Pakistan and the West is how many others could do the same?‬‪

Experts warn that the Pakistani diaspora in the US and the UK could provide eager recruits.