US Secretary of State Condoleezza Rice has arrived in New Delhi as part of efforts to ease tension between India and Pakistan that has surged over the Mumbai attacks.
Pakistan President Asif Ali Zardari said he doubted Indian claims that the only surviving Islamist gunman out of the ten who attacked Mumbai was a Pakistani.
At least 183 people were killed during the three-day siege in India's financial capital.
'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,' Mr Zardari told CNN's Larry King Live, adding that if given evidence his government would take action.
The Pakistan President also signalled he would not accept an Indian demand to hand over 20 of its most wanted men that New Delhi says are living in Pakistan, saying if there was any evidence they would be tried in Pakistan.
Ms Rice made no comments to reporters as she arrived in India.
In other efforts to ease tensions between the two countries, top US military commander Navy Admiral Mike Mullen is visiting the region.
India has long said Pakistan is unable or unwilling to act against anti-India militant groups on its soil.
The latest attacks are threatening to unravel improving ties between the adversaries, who have fought three wars since independence from Britain in 1947.
Mumbai's police chief Hasan Gafoor said the attackers had trained for a year or more in commando tactics.
Azam Amir Kasav, the only surviving gunman, told investigators he is a Pakistani citizen from Punjab, Mr Gafoor said.
Other investigators have said the gunmen were all Pakistanis, from the Lashkar-e-Taiba group.
Indian Foreign Minister Pranab Mukherjee said military action was not being considered but later warned that a peace process begun in 2004 was at risk if Pakistan did not act decisively.
