United States Secretary of State Condoleezza Rice urged Pakistan to take a hard line on terrorism after last week's attacks in Mumbai when she made a hastily arranged visit to Islamabad on Thursday.
In a delicate balancing act, Dr Rice met Indian leaders a day earlier in New Delhi, where she called for restraint in a bid to curb tensions between the nuclear-armed South Asian neighbours.
"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," she told reporters travelling with her from New Delhi to Islamabad.
India has blamed groups based on Pakistani territory for the attack by a band of gunmen which killed 171 people in India's financial capital.
US officials have also blamed groups based wholly or partially in Pakistan.
"Pakistan has to determine its own response here. It just needs to be a robust response and it needs to be effective," Dr Rice said.
"This was a terrible attack and it can't be allowed to happen again," she told Foreign Minister Shah Mehmood Qureshi.
President Asif Ali Zardari told Dr Rice he had asked India to see this as a chance to work together rather than be at odds with one another, saying: "I intend to do everything in my power".
A confrontation between the south Asian rivals would undercut efforts to bring stability to Afghanistan and defeat al Qaeda.
Pakistani security officials have said they could feel compelled to abandon the campaign against Islamist militancy and take forces away from the Afghan border, where they are fighting al Qaeda and the Taliban, and move them to the Indian border if tension increases.
Speaking in New Delhi, Dr Rice said she had gone to India to show US solidarity and empathy with the Indian people.
Pakistan must help India in its investigation "transparently, fully, urgently", she said and also made clear that India should show restraint to avoid fuelling tensions between the neighbours.
