The top US diplomat in Kabul warned that Pakistan posed a bigger security problem for the rest of the world than Afghanistan, in a newspaper interview published on Thursday.
Christopher Dell spoke after Tuesday's attack on Sri Lanka's cricket team as they travelled to a Test match in
Lahore which left eight people dead, and has raised doubts about the government's ability to tackle Islamic militancy.
"From where I sit (Pakistan) sure looks like it's going to be a bigger problem," Dell told the Guardian newspaper.
"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 told the British daily.
Pakistan, a key US power in the "war on terror", is battling Taliban and Al-Qaeda militants along its rugged and lawless border with Afghanistan in the northwest.
More than 1,600 people have died in attacks in Pakistan in the last 22 months and analysts say its security agencies are failing to provide adequate security against militants, who could challenge the rule of President Asif Ali Zardari.