Pakistan poses a 'bigger' security challenge than Afghanistan for the United States and the rest of the world, a top American diplomat has said.

Christopher Dell, who currently runs the US embassy in Kabul, said Pakistani Taliban [Images] groups had formed a common front to attack North Atlantic Treaty Organisation troops in Afghanistan.

"From where I sit (Pakistan) sure looks like it's going to be a bigger problem," Dell told The Guardian in Kabul.

"It is certainly one of those nuclear armed countries the instability of which is a bigger problem for the globe. Pakistan is a bigger place, has a larger population, it's nuclear-armed. It has certainly made radical Islam a part of its political life, and it now seems to be a deeply ingrained element of its political culture. It makes things there very hard," he said.

Dell's comment came in the wake of the terrorist attack on the visiting Sri Lankan cricket team in Lahore [Images] on Tuesday.

According to the report, senior officials in the Foreign Office and the Obama [Images] administration have privately expressed concern that Pakistan could prove to be more of a danger to global peace and security in the long run than Afghanistan, because of its nuclear weapons and its highly politicised secret service, the Inter-Services Intelligence agency.