Indian Prime Minister Manmohan Singh says Sikh separatists based outside India are trying to revive militancy in the state of Punjab.
Mr Singh says there is "credible information" to show that the remnants of separatist groups in Canada, the UK, Germany and Pakistan are regrouping.
He said they were doing so with the help of other "hostile forces".
Indian security forces battled a bloody uprising by Sikh secessionists in Punjab in the 1980s.
Prime Minister Indira Gandhi was assassinated in 1984 by her Sikh bodyguards in apparent retaliation for ordering an army attack on the Golden Temple, Sikhism's holiest shrine.
'Extremist elements'
Mr Singh, a Sikh himself, expressed concern over "die-hard separatists" in a letter to the head of Sikh temples, Avtar Singh Makkar.
"The government and our agencies have credible information of efforts being made by extremist groups to revive militancy in Punjab," his letter stated.
Referring to last year's bomb blasts in two cinema halls in Ludhiana, Punjab, Mr Singh said the two people who planned it were "induced" to carry out the attack on a visit to Pakistan with funding from "extremist elements in the US".
There is growing concern in India that former members of extremist groups are indeed regrouping, as Mr Singh fears.
Former members of one such group, Babbar Khalsa International, are said to be active in Punjab.
Violent protests which broke out last year in the state against the head of a sect, Dera Sachcha Sauda, were said to have been instigated by this group.
Indian intelligence agencies believe Babbar Khalsa has the backing of Pakistan's spy agency, the Inter Services Intelligence (ISI) agency.
The ISI has long been accused by Delhi of supporting armed separatists in Indian-administered Kashmir and the country's north-east.
Echoing concern over developments in Punjab, National Security Adviser MK Narayan last year warned that there had been "a manifest attempt in Pakistan to build up a radical Sikh environment".
Mr Narayanan said there were "signs of resuscitation of militant groups in Canada, the US and Germany".
Punjab, called the granary of India, is economically one of the leading states.
The state has been largely peaceful since 1993.
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