US Secretary of State Condoleezza Rice has called on the Pakistani government to take a "tough line" on terrorism after arriving in Islamabad for talks.
Ms Rice also said Pakistan had to mount a "robust" response to last week's attacks in the Indian city of Mumbai, which left at least 188 people dead.
India says the attackers had links to Pakistan, which Islamabad has denied.
Ms Rice arrived in Pakistan's capital after visiting India amid growing tension between the two neighbours.
In Delhi on Wednesday, she called on both countries to show moderation in their response to the Mumbai attacks.
'Robust and effective'
"The global threat of extremism and terrorism has to be met by all states, taking a very tough and hard line, and so that is what I am going to discuss," Reuters news agency quoted Ms Rice as saying.
"Pakistan has to determine its own response here. It just needs to be a robust response and it needs to be effective," she said.
Ms Rice landed at the military Chaklala air base in Islamabad direct from India.
India says the attackers came from Pakistan
During her one-day visit, she is scheduled to meet Pakistan's President Asif Ali Zardari and Prime Minister Syed Yusuf Raza Gillani to discuss the Mumbai attacks.
In Delhi, Ms Rice said that Pakistan must act "fully and transparently" in efforts to bring the Mumbai attackers to justice.
"Pakistan has a special responsibility to do so," she told a press conference.
Ms Rice was speaking after meeting Indian Foreign Minister Pranab Mukherjee.
"This is the time for everybody to co-operate and do so transparently, and this is especially a time for Pakistan to do so," she said.
Ms Rice also warned India not to take actions that would provoke "unintended consequences".
"Any response needs to be judged by its effectiveness in prevention," she said
Last week's attacks at multiple locations in Indian's financial capital stunned the country, with many describing it as India's 9/11.
Indian authorities have said that the attackers came from Pakistan.
The Indian government has been under tremendous pressure domestically to take action against those responsible for the attack.
On Wednesday, thousands of people protested in Mumbai, blocking traffic and shouting slogans, including "Down with Pakistan" and "Shame on politicians".