Manmohan Singh said Pakistan had given "sanctuary to terrorists"
November's attack on Mumbai must have had support from some official agencies in Pakistan, Indian Prime Minister Manmohan Singh has said.
Mr Singh also accused Pakistan of "whipping up war hysteria" in what correspondents say may be the toughest comments yet by a senior Indian figure.
Pakistan rejected Mr Singh's allegations and accused India of raising regional tension.
More than 170 people died when 10 gunmen targeted Mumbai.
Separately, the sole surviving gunman from the raid, Mohammed Ajmal Amir Qasab, was remanded in custody on Tuesday for a further two weeks until 19 January.
Speaking to a meeting of the chief ministers of Indian states in Delhi, Mr Singh said that because of the "sophistication and military precision of the attack it must have had the support of some official agencies in Pakistan".
At least 173 people were killed in the attacks
He added: "Today, even as Pakistan engages in whipping up war hysteria, our nation remains steadfastly united and, if anything, the process of national consolidation is becoming stronger."
Mr Singh again criticised Pakistan's policies on tackling terrorism, saying it had "given sanctuary to terrorists and other forces who are antagonistic to India".
He added: "The more fragile a government, the more it tends to act in an irresponsible fashion."
Mr Singh said India must convince the world that states which use terrorism as an instrument of foreign policy must be isolated. Soft support for terrorism, he said, can no longer be endorsed.
Pakistan said Mr Singh's comments only served to raise tension in the region.
"The government of Pakistan emphatically rejects the unfortunate allegations levelled against Pakistan by the prime minister of India," the foreign ministry said in a statement.
Gunman Mohammed Ajmal Amir Qasab is again remanded in custody
"Instead of responding positively to Pakistan's offer of co-operation and constructive proposals, India has chosen to embark on a propaganda offensive," it said, adding that such an approach was "fraught with grave risks".
Some Pakistani analysts say the open and public accusation by India against official bodies in Pakistan is not likely to increase Islamabad's inclination to co-operate.
India says the men who carried out the attack were from the Pakistan-based militant group Lashkar-e-Taiba (LeT), which the BBC's Chris Morris in Delhi says has longstanding links with Pakistan's top spy agency, the Inter-Services Intelligence (ISI).
Both LeT and Pakistan have denied any involvement in the attacks.
On Monday, Delhi handed to Islamabad evidence on the attacks said to include the interrogation of the surviving gunman, details of phone conversations between the attackers and weapons information.
Shahid Malik, the Pakistani High Commissioner in Delhi, said on Tuesday: "India has given us some material, we are examining it. There is no question of rejection or otherwise."
On Monday Pakistan's Prime Minister Yousuf Raza Gilani repeated a promise that it would punish any of its citizens if "credible" evidence were found of their involvement in the attacks.
But Pakistan described Indian accusations that Pakistani "state actors" were involved in the attacks as "speculation".
US Assistant Secretary of State Richard Boucher visited Pakistan on Monday and urged the two rivals to co-operate more on the investigation.
Gunmen stormed two Mumbai hotels, a crowded railway station, a Jewish centre and a popular restaurant on 26 November.
The attacks lasted three days and left at least 173 people dead.
The surviving gunman has now been remanded for a third time.
Mohammed Ajmal Amir Qasab faces a number of charges including murder, attempted murder, waging war against a country and criminal conspiracy.
Police say he and an accomplice, Ismal Khan, opened fire indiscriminately at the CST station and two other places, killing more than 50 people, including three top police officers.
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