D.M. Jayaratne said an unknown number of Liberation Tigers of Tamil Eelam (LTTE) fighters were based at secret camps in the south Indian state of Tamil Nadu.
"We have intelligence reports of three clandestine training centres operated by the LTTE in Tamil Nadu," the prime minister said in a statement on Thursday.
Jayaratne said the rebels, who were defeated by Sri Lankan government troops in May 2009, were hoping to relaunch their decades-long fight for an independent homeland.
"Their next target is to create small-scale attacks," Jayaratne said. "The entire nation must be ready to face this threat."
The Indian government immediately rejected the claims.
"We categorically deny existence of any LTTE camps in India," foreign ministry spokesman Vishnu Prakash said on Twitter. "We urge Sri Lanka to desist from reacting to speculative and uncorroborated reports."
Sri Lanka has suffered no rebel attacks since the Tigers were wiped out, but Jayaratne told parliament on Wednesday that the country needed to maintain tough emergency laws to deal with their possible resurgence.
Colombo has resisted international calls to end the emergency laws, which are routinely approved by parliament.
The United Nations estimates that up to 100,000 people died in the ethnic conflict which lasted from 1972 until 2009.
The opposition accuses the government of using emergency laws to stifle political dissent and the media, charges denied by the authorities. (*)
Editor: Kunto Wibisono
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