US 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, 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," Rice 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, including six Americans, in India's commercial capital. US officials have also blamed groups based wholly or partially in Pakistan.
"Pakistan has to determine its own response in Islamabad. It just needs to be a robust response and it needs to be effective," 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 Rice he had asked India to see this as a chance towork together rather than be at odds with one another, saying: "I intend to do everything in my power".
"The government will not only assist in investigations but also take strong action against any Pakistani elements found involved in the attack," a statement quoted Zardari as saying.
"Pakistan is determined to ensure that its territory is not used for any act of terrorism," Zardari said.
Before meeting Zardari and Prime Minister Yousaf Raza Gilani, the leaders of an eight-month-old civilian government in Islamabad, Rice first met army chief General Ashfaq Kayani at army headquarters in nearby Rawalpindi.
Traffic was blocked and no people were in sight aside from security personnel lining roadsides as Rice's motorcade passed through two cities living under constant threat of attacks by militants linked to the Taliban and al Qaeda.
How much leverage the United States, particularly the outgoing Bush administration, has over Pakistan is debatable. Withholding financial or military support could add to instability in the Muslim state.
'CONTROLLERS IN PAKISTAN'
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, 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.
She also made clear that India should show restraint to avoid fuelling tensions between the neighbours, who have fought three wars since independence from Britain in 1947.
"This is the time for everybody to cooperate," she told a news conference with Indian Foreign Minister Pranab Mukherjee.
Mukherjee had harsh words for Pakistan.
"I informed Dr. Rice there is no doubt that the terrorist attacks in Mumbai were perpetrated by individuals who came from Pakistan and whose controllers are in Pakistan," he said.
Rice's meeting with Kayani was significant as Lashkar-e-Taiba, the prime suspect in the Mumbai attack, is a jihadi organisation that analysts say has in the past had ties to the Pakistani military's Inter-Services Intelligence agency.
With Pakistan in the midst of a fragile transition to democracy after more than eight years of rule under former army chief Pervez Musharraf, analysts say the new government does not have full control over the army's affairs.
Arriving in Islamabd, Rice said the onus was on the Pakistani leadership "as a whole" to tackle the threat of terrorism.
Rice's visit followed one a day earlier by the chairman of the US Joint Chiefs of Staff, Admiral Mike Mullen. The two met for breakfast in New Delhi to compare notes.
In a thinly veiled reference to Kashmir separatists suspected of carrying out the attack on Mumbai, Mullen also encouraged Pakistan to act against jihadi groups everywhere, not just in regions bordering Afghanistan, where Pakistani forces have been fighting tribal militants, the Taliban and al Qaeda.
Pakistan has condemned the assault, denied any involvement by state agencies and vowed to help the Indian investigation, but wants tangible proof of Pakistani involvement.
Pakistan has also indicated it will not accept an Indian demand to hand over 20 most-wanted men said to be living there.