Thousands have protested in Mumbai over the attacks
US Secretary of State Condoleezza Rice has 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 reporters in Delhi.
India says the attackers, who killed at least 188 people, have links to Pakistan, which denies any role.
Meanwhile thousands of people have held a rally in Mumbai in protest at the attacks, many angry at the Indian government for failing to prevent them.
Separately, Mumbai's police said they had defused explosives left by militants in the main train station.
Last week's attacks at multiple locations in Indian's financial capital stunned the country, with many describing it as India's 9/11.
'Time to co-operate'
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," Ms Rice said.
Condoleezza Rice calls on Pakistan to co-operate with investigations
She 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.
Analysts say she is worried that an escalation of military tension in South Asia could distract Pakistan from the battle against militants on its Afghan border and simultaneously undermine its civilian government.
Mr Mukherjee said at the joint press conference with Ms Rice that there was "no doubt" that the militants had come from Pakistan and were co-ordinated from Pakistan.
As Ms Rice visited Delhi, the chairman of the US Joint Chiefs of Staff Mike Mullen was visiting Pakistan.
He urged Pakistan to broaden its campaign against militant groups following the Mumbai attacks.
Adm Mullen called on Islamabad to "take more, and more concerted, action against militant extremists elsewhere in the country".
Six Americans died in the Mumbai attacks.
The Mumbai protest drew thousands of people who blocked traffic and shouted slogans, including "down with Pakistan" and "shame on politicians".
Two attackers at the car park
Trader Mahesh Patel told Reuters news agency: "I have come with my friends because we cannot take it any more. The politicians must act, they must stop taking us for granted."
Earlier police said they had found explosives hidden in a bag in Mumbai's main train station, which they said were left over from last week's attacks.
The BBC's Damian Grammaticas in Mumbai says the discovery will again raise more questions about Indian security failures.
Officials have also released a previously unseen video of attackers in the car park of Mumbai's main train station.
Pakistan's political parties earlier joined forces to sign a resolution saying they shared India's grief after the Mumbai attacks and abhorred violence against innocent people.
Jihadi organisation based in Pakistan
Formed towards the end of the Afghan war against the Soviets
Blamed for hundreds of attacks in the region since 1990
But the parties also said they took strong exception to what they called "unsubstantiated allegations made in haste against Pakistan".
Mr Mukherjee said a military response was not under consideration but that if Pakistan did not act, the bilateral peace process would be at risk.
India has also asked Islamabad to hand over 20 fugitives from Indian law it believes are hiding in Pakistan.
However, Pakistani President Asif Ali Zardari indicated there would be no handover.
He told US television: "If we had the proof, we would try them in our courts, we would try them in our land and we would sentence them."
One militant was captured alive from the Mumbai attacks.
Azam Amir Qasab is in police custody and Indian police have said he is "certainly" from Pakistan.
Mumbai joint police commissioner of crime Rakesh Maria told CNN the militant had spent the past 18 months at camps run by militant group Lashkar-e-Toiba and was trained by ex-army officers in Pakistan-administered Kashmir.
However, Mr Zardari said: "We have not been given any tangible proof to say that he is definitely a Pakistani. I very much doubt... that he's a Pakistani."
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