The United States said the Mumbai attacks had shown that the Lashkar-e-Tayiba was out to "fill the gap" created by a "diminished Al-Qaeda" and a "decisive" action was needed from all countries of South Asia, including Pakistan, to defeat it.
US Coordinator on Counter-Terrorism Dan Benjamin said the aim of securing the world would remain incomplete unless LeT is defeated.
"I have said that we will not achieve our security aims if this group (LeT), with thousands of men under arms (is active)... We could see in Mumbai attacks, the target was set directly out of Bin Laden's book, filling the gap created by a diminished Al Qaeda," Benjamin said at a function in New Delhi.
"We should be clear that terrorist threat in South Asia requires decisive counter-terrorism efforts by all countries of the region," Benjamin added.
He said his country was not just focussing on Al Qaeda "but also against groups which are growing and have global reach like LeT".
The US official said the US and allied forces have made important gains against Al Qaeda but socio-political and economic issues were to be addressed to counter the challenge thrown by terrorism.
Benjamin rejected the contention by some sections in Pakistan that Jamaat-ud Dawa (JuD) was a humanitarian group.
"If any organisation has a political, a humanitarian and a terrorist wing, then it is entirely a terrorist group, like the LeT and its social service wing, the Jamat-ud-Dawa," the US official said.
He said one of the major challenges was to pre-empt such groups providing essential social services in countries, which have insufficient resources.
Contending that Pakistan had "suffered tremendously" due to terrorism, he said the US will continue to support the country in strengthening its democratic institutions, economic growth and defeat extremists.
"Pakistan has suffered tremendously from terrorism in recent years and lost thousands of lives in the last couple of years alone. With our cooperation their performance in Swat demonstrates that it's continuing to beat back militancy," Benjamin said.
He said that while seeking cooperation of all nations in the South Asian region, "We all need to avoid the kind of thinking that working with one country for countering terrorism is not at the expense of the other nation."
Citing "lot of history" in South Asia, he said one has to be mindful of it but "cannot be prisoners to it as we deal with a new kind of threat precisely because your success will depend on how well you work together".
He also insisted that India was in the loop in the US' Af-Pak policy. "India is not out of the process. It is very much involved (in the Af-Pak process)."