US Secretary of State Condoleezza Rice has said India, Pakistan and others must act with "urgency and resolve" to bring the Mumbai attackers to justice, reported BBC.
"This is especially a time for Pakistan to do so," she told reporters in Delhi, ahead of talks with Indian leaders.
Ms Rice is in India to ease tensions between India and Pakistan after last week's attacks which killed nearly 200.
India says the attackers had Pakistani links. Islamabad denies any role and has offered to help the investigation.
US officials have said India was warned of a possible threat a month before the attacks.
Meanwhile, political leaders in Pakistan joined to condemn the attacks but rejected what they called "hasty allegations" made against Pakistan.
Last week's attacks at multiple locations in Indian's financial capital stunned the country, with many describing it as India's 9/11.
"This is the time for everybody to co-operate and do so transparently," Ms Rice told a news conference at the US embassy in Delhi.
"I have said that Pakistan needs to act with resolve and urgency and co-operate fully and transparently. That message has been delivered and will be delivered to Pakistan."
She said the US would not "jump to conclusions" about who was responsible, but added: "This is clearly the kind of terrorism in which al-Qaeda participates."
Ms Rice is to meet India's prime minister and foreign minister. No plans have been announced for a visit to Pakistan.
However, the chairman of the Joint Chiefs of Staff, Adm Mike Mullen, has arrived in Islamabad for talks with the Pakistani government and military.
It was not clear whether Ms Rice would visit Mumbai. Six Americans died in the attacks.
Pakistan's political parties have joined forces to sign a resolution saying they shared India's grief after the Mumbai attacks and abhorred violence against innocent people.
But the parties also said they took strong exception to what they called "unsubstantiated allegations made in haste against Pakistan".
Indian Foreign Minister Pranab 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 asked Islamabad to hand over 20 fugitives from Indian law it believes are hiding in Pakistan.
But 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."
On Tuesday, a US official told the Associated Press that the Indian authorities had been told of an apparent plot to launch an attack on Mumbai from the sea.
Smoke billows from the Taj Mahal Palace hotel in Mumbai, India
Anger is growing in India that the attacks were not prevented
Mumbai police chief Hassan Gafoor later appeared to confirm the report by saying that there had been a known threat to at least some of the locations targeted, including the Taj Mahal Palace hotel.
One of the gunmen, named as Azam Amir Qasab, survived and is in police custody. Police have said he is "certainly" from Pakistan.
But Mr Zardari, on his interview on CNN's Larry King Live, 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."
The Indian media have said he is linked to Lashkar-e-Toiba, but the group has denied responsibility.
US Director of National Intelligence Mike McConnell implied Lashkar-e-Toiba was to blame for the attacks.