US Chairman Joint Chief of Staff Committee Admiral Mike Mullen said that Pakistan has done a lot in the fight against terrorism.
He was speaking in an interview with an Indian news channel.
Admiral Mike Mullen has arrived in Islamabad, where he is expected to meet President Asif Ali Zardari, Prime Minister Yousuf Raza Gilani and Chief of Army Staff General Ashfaq Pervez Kayani.
Both sides will discuss several issues including war on terror, the attacks on Nato-led troops in Pakistan and the implementation on the agendas discussed in the Kabul Conference.
On the eve of his one-day visit, Mullen acknowledged worries over extremist sanctuaries in Pakistan, including the Haqqani network based in North Waziristan.
Mullen, speaking at a press conference on Friday in New Delhi, called the Haqqani militants "the most lethal network" faced by the US-led international force in Afghanistan and said he has repeatedly urged Pakistan to tackle the threat.
"I continue to address that with the (Pakistani) leadership as a very, very strong priority," Mullen told reporters at the end of a two-day visit to India. Pakistan's army chief General Ashfaq Kayani "is aware of that priority and in terms of what we think needs to happen," Mullen said.
Mullen also said Pakistan's intelligence service needed to change its outlook. "I believe the strategic approach, the overall strategic approach of ISI needs to fundamentally change," he said.
US officials have praised Pakistan for moving against militants in the northwest but have urged the military to expand its operations to include the Haqqani network.
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