US Secretary of State Condoleezza Rice flew from India to Pakistan on Thursday on a hastily arranged visit aimed at defusing tension between the two rival nations after the Mumbai attacks.
During her visit to India, Rice was told by Foreign Minister Pranab Mukherjee that India was considering all options in dealing with Pakistan after the terror assault, which left nearly 200 dead.
Mukherjee said there was "no doubt" that the militants had come from and were coordinated from Pakistan.
"What action will be taken by the government will depend on the response we have from the Pakistan authorities," he said, referring to India's demand that Pakistan hand over 20 terrorist suspects.
"Whatever the government considers necessary to protect its territorial integrity, safety and security of its citizens, the government will do that," he warned.
The attacks against India's economic capital -- which began last Wednesday and lasted 60 hours -- were carried out by 10 gunmen, some of who arrived by boat.
Indian security forces detained one of the militants alive, and officials say he has admitted to being a Pakistani from Lashkar-e-Taiba, an Islamist group based across the border and long seen as a creation of Pakistan's shadowy spy service.
The targets included two luxury hotels, a Jewish centre, a popular cafe, a packed railway station and a hospital. The official toll of victims stands at 188 dead -- among them 22 foreign nationals -- and more than 300 wounded.
Although Rice said she refused to "jump to any conclusions about who is responsible," US intelligence officials have supported India's accusations.
"Pakistan needs to act with urgency and with resolve and cooperate fully and transparently," said Rice during her emergency one-day trip to India.
"The response of the Pakistan government should be one of cooperation and action. That is what we expect and we have been sending that message," said Rice.
Ties between India and Pakistan have been badly strained by the attacks, with New Delhi reportedly considering suspending the ongoing peace process.
The two countries have fought three wars since their 1947 independence from British rule, and both have nuclear weapons.
The chief of the United States military, Admiral Michael Mullen, travelled in the opposite direction to Rice, arriving in India for talks on Thursday after visiting Pakistan.
He urged Pakistani leaders, including President Asif Ali Zardari, "to investigate aggressively any and all possible ties to groups based in Pakistan," a US embassy statement said.
Rice and Mullen had a breakfast meeting at the US ambassador's residence in New Delhi before she left for Islamabad, diplomats said.
India remains on high alert a week after the attacks, with Indian police on Wednesday discovering and defusing more explosives left behind by the highly trained gunmen at Mumbai's main railway station.
Pakistan has suggested setting up a "joint investigation" but says it wants concrete proof that all the attackers were Pakistanis.
India hauled in the Pakistani ambassador earlier this week and demanded Pakistan arrest and extradite 20 terror suspects, including the founder of Lashkar-e-Taiba, Hafiz Saeed.