Bombay, India ? The bombers who targeted Bombay’s rail system had support from inside Pakistan, India’s prime minister said Friday, warning that the nuclear-armed rivals’ peace process could be derailed unless Islamabad reins in terrorists.

Prime Minister Manmohan Singh’s unusually blunt comments appeared to signal a major shift in relations between India and Pakistan, whose ties had warmed over the past two years.

“We will leave no stone unturned – I reiterate, no stone unturned – in ensuring that terrorist elements in India are neutralized and smashed,” Singh told reporters in Bombay. “These acts of terrorism are desperate acts of desperate individuals.”

Singh, who met bombing victims and officials, noted that Pakistan had assured India two years ago its territory “would not be used to promote, encourage, aid and abet terrorism.

“That assurance has to be fulfilled before the peace process and other processes progress,” he said.

Pakistan quickly denied Singh’s accusations, with Foreign Ministry spokeswoman Tasnim Aslam calling them “unsubstantiated.”

However, Singh said investigators are certain that terror cells operating in India “are instigated, inspired and supported by elements across the border, without which they cannot act with such devastating effect.

“They clearly want to destroy our growing economic strength,” Singh said.

After coming to the brink of war in 2002, India and Pakistan began a peace process that has brought them closer, yet concrete agreement on the most pressing issue – the Himalayan region of Kashmir – has been minimal.

Scheduled talks July 20 between the foreign secretaries of the two countries appeared increasingly unlikely, with local news reports saying they had been canceled. Navtej Sarna, the Indian Foreign Ministry spokesman, said only that no announcement had been made.

In an indication of the growing criticism of the government, a phone-in survey Thursday by the NDTV news channel found that 99 percent of callers saying the government was too soft on terrorism. While clearly unscientific – and apparently designed to stoke public outrage – the results nonetheless underscored the pressure on officials.

Investigators were casting a wide net for the assailants – focusing on a Pakistan-based Islamic militant network, Lashkar-e-Tayyaba, along with smaller homegrown groups.

The inquiry spread to Nepal, where police said Friday they had arrested two Pakistanis in connection with the seizure of plastic explosives in Katmandu in 2001. They said the men are being investigated for links to the Bombay blasts.

Kashmir, which is divided between India and Pakistan but claimed by both, is at the heart of their rivalry. The countries have fought three wars since Britain divided colonial India into India and Pakistan in 1947, two of them over Kashmir.