Without taking any country's name, Prime Minister Manmohan Singh on Friday said the serial blasts in Mumbai were carried out by terrorists with support from across the border.
He warned Pakistan that it would be "exceedingly difficult" to carry forward the peace process if it did not control terrorist elements operating from its territory.
"I have explained it to the government of Pakistan at the highest level that if the acts of terrorism are not controlled, it is exceedingly difficult for any government to carry forward what may be called as normalisation and peace process," he told a press conference in Mumbai after a three-hour visit to the city rocked by Tuesday's blasts in trains that killed about 200 people.
"We are also certain that these terror modules are instigated, inspired and supported by elements across the border without which they cannot act with such devastating effects," he said.
Asked whether talks on confidence-building measures with Pakistan will continue, Singh said, "Pakistan, in 2004, had solemnly given an assurance that Pakistani territory will not be used to promote, encourage, aid and abet terrorist elements directed against India."
"That assurance has to be fulfilled before the peace process or other processes can make progress," he said.
Noting that the terrorists were intending to destroy India's economic strength, unity and communal harmony, he asserted, "We cannot allow this to happen."
Earlier, Singh visited hospitals where those injured in the blasts are being treated and met top officials of the state government.
"The terrorists (responsible for the blasts) were supported by elements across the border without which they cannot hit with such an effect," Singh said without naming any country.
Noting that the terrorists were intending to affect India's economic progress, the Prime Minister asserted, "We cannot allow this to happen."
He said "it is time to crack down to destroy all these anti-national elements" and that "no stone will be left unturned" in ensuring that terrorism is eliminated.
Singh said terrorists, by carrying out such attacks, wanted to create communal tension in India. He asserted that terrorist elements could not be allowed to destroy the country's secular fabric.
Terrorists are acquiring sophistication and in view of this there is a need to upgrade the technology available to the security forces, the Prime Minister said.
He said steps like installation of sophisticated surveillance gadgets at vital installations and high profile establishments would be taken.
"The whole nation admires the spirit of Mumbai," the prime minister said.
"Just as things can never be the same for those who have lost their near and dear ones, it cannot be dismissed as usual for us," he said.
Contending that "we have to ponder over and work out an effective strategy that such things do not recur", he said, "We must recognise that the terrorists are trying to spread their tentacles across the country."
Earlier, Singh, who arrived on Friday afternoon, interacted with top Maharashtra officials at the airport. The meeting was also attended by Chief Minister Vilasrao Deshmukh and Union Agriculture Minister Sharad Pawar.
Singh reviewed the progress of investigations into the attacks and took stock of security measures taken by the state government following the explosions.
He visited Sion and KEM Hospitals and spent about 20 minutes there, meeting the injured and inquiring about their condition and their treatment.