NEW DELHI/ISLAMABAD India blamed "elements" from Pakistan on Friday for the assault on its financial capital, Mumbai, raising the prospect of a breakdown in peace efforts between the nuclear-armed rivals.
But Pakistan said it was not to blame, and in an unprecedented step, agreed to send the head of its military's Inter Services Intelligence (ISI) agency to India to share information.
The attacks on two luxury hotels and other sites around Mumbai were mounted after Pakistani President Asif Ali Zardari, widower of assassinated former prime minister Benazir Bhutto, had made bold moves to improve ties with India.
On Thursday, Indian Prime Minister Manmohan Singh pinned the blame for the Mumbai attacks on militant groups based in India's neighbors, usually an allusion to old rival Pakistan.
Foreign Minister Pranab Mukherjee was more explicit.
"Preliminary evidence, prima facie evidence, indicates elements with links to Pakistan are involved," Mukherjee told a news conference in New Delhi on Friday.
He urged Pakistan to dismantle the infrastructure that supports militants.
The two countries have fought three wars since their independence in 1947 and went to the brink of war again after a December 2001 a militant attack on India's parliament that India linked to Pakistan.
Pakistani leaders were quick to condemn the Mumbai attacks and on Friday they denied involvement.
Zardari, in a meeting with Germany's ambassador, said the roots of terrorism lay in Afganistan's war against Soviet occupation in the 1980s.
"The germs of terrorist elements were not produced in security agencies' labs in Pakistan," the Foreign Ministry cited Zardari as saying.
"Pakistan could neither gain anything nor does the democratic government believe in such tactics," he said.
Pakistan's Foreign Minister Shah Mehmood Qureshi, in India on a visit that was scheduled before the attacks, said anti-terrorism cooperation must be strengthened. He called on India not to play politics.
"This is an international network. It has to be dealt with at an international level," he told reporters.
Earlier, Zardari telephoned Singh to condemn the attacks and rule out a link to his government.
"Non-state actors wanted to force upon the governments their own agenda but they must not be allowed to succeed," Zardari's office quoted him as saying.
Pakistan for years supported militants battling Indian forces in the disputed Kashmir region but began to rein them in after the September 11 attacks on the United States.
Taliban and al Qaeda militants have carried out a series of bomb attacks, most on political leaders and the security forces, including the ISI. But analysts say security agents still have links with some Kashmiri militants.
The use of heavily armed "fedayeen" or suicide attackers in Mumbai bears the hallmarks of Pakistan-based militant groups such as Lashkar-e-Taiba or Jaish-e-Mohammed, blamed for the 2001 attack on India's parliament.
Both groups are outlawed in Pakistan. They made their name fighting Indian rule in disputed Kashmir and were closely linked to the ISI.
Lashkar-e-Taiba denied any role in the Mumbai attacks, and said it had no links with any Indian group. Instead, the little-known Deccan Mujahideen claimed responsibility.
Inevitably, though, suspicion will focus on Pakistani militants.
"There is no definitive evidence so far," said London-based political analyst and author, Farzana Shaikh.
"What we have is a great deal of speculation but it wouldn't be entirely implausible, given that sections of Pakistan's intelligence agencies have in the past been linked to terrorist attacks."
Relations between India and Pakistan have warmed in recent years despite no progress on Kashmir.
Zardari has stressed the importance of good ties and said recently Pakistan was willing to commit to a policy of no first use of nuclear weapons and suggested a pact.