Indian Prime Minister Manmohan Singh warned yesterday the peace process with Pakistan could be harmed by the Mumbai train bombings and alleged the attackers had support "from across the border".

The prime minister, on his first visit to Mumbai since the blasts that killed 179 and injured almost 800, warned the peace process would struggle to advance unless Pakistan cracked down on militants operating from its soil.

"The terrorists [responsible for the blasts] were supported by elements across the border without which they cannot hit with such an effect," Singh said.

"I explained to the government of Pakistan ... that if such acts of terrorism are not controlled, it is exceedingly difficult for any government to carry forward what may be called the normalisation of relations and the peace process," he said.

Senior police in Mumbai say Tuesday's rush-hour bombings bear the hallmarks of Lashkar-e-Taiba, a pro-Pakistan Islamist group based in divided Kashmir that has been blamed for previous attacks in India.

Meanwhile, a news agency reported that high-level talks between India and Pakistan scheduled for July 20-21 are "unlikely" to take place.

Press Trust of India news agency quoted "official sources" - a euphemism for top government officials - as saying that there was little possibility of the talks taking place.

The talks were scheduled to take place on July 20 between the foreign secretaries - the top civil servants in the ministries of the two countries - to review the progress of the peace process between the two nuclear rivals. It was to be followed by a meeting of the foreign ministers on July 21. Anti-terrorist police, under pressure to produce a breakthrough, said they were pursuing "important leads" but added the identity of the group behind Tuesday's blasts remained "hazy".

"We're exploring every single angle. We have some pictures in view but it's very hazy," additional commissioner of police Jayjit Singh said. Indian police tracked telephone calls to Pakistan, Bangladesh and the Middle East yesterday in their search for the culprits behind the bombings. A telephone call traced from Karachi to Dhaka after the blasts said "Congratulations", The Times of India reported.

The newspaper said another caller contacted his mother in Karachi from a public call box in Mumbai saying he was fine but could not disclose his location, and swiftly hung up.

Pakistan dismissed the report as "frivolous" and repeated President Pervez Musharraf's offer to help the inquiry.

Analysts believe the bombers hoped to fuel Hindu-Muslim tensions and may have targeted first-class carriages to maximise casualties among wealthy Hindus.

Senior investigators said two suspected militants who escaped a police arms swoop in a city close to Mumbai in May, when arms and explosives were seized, could know something about the attacks. "It's quite possible they might have some knowledge ... but they are not the people we are focused on for this case. They are not the masterminds," head of the inquiry team K.P. Raghuvanshi said.

The Press Trust of India named a man, Rahil, as a potential planner, saying he had affiliations with Lashkar-e-Taiba and was thought to be behind bus and market bombs in the capital last year that killed more than 60 people.

Media reports claim Mumbai-based activists from the banned Students Islamic Movement of India (SIMI) were suspected of having a role in the attacks.

Both Lashkar-e-Taiba and SIMI have both denied any part in the blasts.