Indian Prime Minister Manmohan Singh has said that Pakistan needs to curb terrorism if the peace process between the two countries is to make progress.
Manmohan Singh visiting bomb victims on Friday
But Islamabad has rejected as "unsubstantiated" Mr Singh's claim that the Mumbai train bombers received help from within Pakistan.
Nearly 180 people died in the blasts.
"We are also certain that ... terrorist modules are instigated, inspired and supported by elements across the border," Mr Singh said.
The BBC's Sanjoy Majumder in Delhi says that Mr Singh's comments represent a hardening of positions against Pakistan for the first time since Tuesday's attacks.
With the investigations making little tangible progress, our correspondent says the Indian government is under increasing pressure to act and demonstrate that it is not a soft target.
Talks between senior Indian and Pakistani civil servants scheduled for next week are unlikely to go ahead, according to high-level sources in Delhi. There has been no official confirmation that the discussions have been put off.
Pakistan denies involvement in the Mumbai blasts and issued a swift rebuttal to Mr Singh's comments.
"These allegations are unsubstantiated, we have already rejected them," said foreign ministry spokeswoman Tasnim Aslam.
Pakistani President Pervez Musharraf has condemned the loss of "precious lives" in the attacks, in which seven commuter trains were bombed in less than 15 minutes.
But in the days since, there have been sharp exchanges between government ministers in India and Pakistan.
'No stone unturned'
Mr Singh said on Friday that "acts of terrorism" were "despicable acts of desperate people".
He was speaking after visiting victims of the blasts.
"We will leave no stone unturned in ensuring that terrorist elements in India are neutralised," he told journalists at a news conference after visiting bomb victims.
Suspicions that Pakistan-based militants may have been involved have raised tensions between the two states.
Indian security officials have suggested that the Mumbai bombings bore the hallmarks of Lashkar-e-Toiba, a Kashmiri militant group operating from Pakistan.
But the Indian government has not directly accused the group, which has denied any responsibility.
A Muslim organisation banned in India, the Students' Islamic Movement (Simi), has also denied involvement in the attacks.
Sketches of three men wanted over the bombings have been published in the Indian media, but officials have admitted they have been making slow progress with investigations.
Prime Minister Singh flew into Mumbai on Friday amid tight security and immediately drove to the Sion hospital where he met some of the blast victims.
"The prime minister was saying to the patients 'what you want, I will give you,'" Agnes George, who was visiting one of the injured, told the AFP news agency.
Nuclear rivals Pakistan and India began peace talks in January 2004.
They followed a prolonged period of tension after militants attacked the Indian parliament in December 2001. That attack led to both sides amassing troops along their border.
Although both sides have made some headway in improving travel and other ties between them, there has been little sign of progress in solving their core dispute over the divided territory of Kashmir, which both countries claim.