US said it was Pakistan's "duty and obligation" to bring the perpetrators of the 2008 Mumbai attacks to justice as quickly as possible.
Robert Blake, Assistant Secretary for South and Central Asian Affairs, said the US has always taken LeT, the terror outfit which carried out the 26/11 attacks, seriously.
"We have already taken them extremely serious and now American counter-terrorism experts identify LeT as probably the single most serious counter-terrorist threat after the Al-Qaeda," he said.
Blake said President Barack Obama was "very forthright" about LeT's threat during his recent India visit.
"It is Pakistan's duty and obligation to cooperate to bring bring those who are responsible for the terrible 26/11 attacks to justice and to do so as quickly as possible. Pakistan must address the problem," he said.