The Sri Lankan government today announced it was stopping its military pressure against Tamil separatists and some of the militant Tamil guerrillas agreed to go along with the cease-fire, opening the way for a possible political settlement to the island nation's bloody civil war.

To show its good faith, the government of President Junius R. Jayewardene also said it would relax security restrictions against the Tamil minority, such as night curfews and prohibited zones, depending on the success of the cease-fire.

The easing of military activities by the government, which Sri Lanka's National Security Minister Lalith Athulathmudali called a "cessation of hostile incidents," follows a meeting here two weeks ago between Indian Prime Minister Rajiv Gandhi and Jayewardene.

The Sri Lankan Tamils are related ethnically and religiously to Tamils in India, who have put political pressure on Gandhi to protect the Tamil minority from Sri Lanka's Sinhalese majority. Tamils are largely Hindus; the Sinhalese are mostly Buddhists.

In the southern Indian city of Madras, where many Indian Tamils live and where Sri Lanka has claimed that Tamil fighters are given refuge and training, four major militant groups agreed to the cease-fire. But there are about 35 different separatist groups, and at least one of them refused to comply with the government's request to stop fighting and start negotiating.

The announcement that the Sri Lankan government was stopping its military actions was made by Athulathmudali in Colombo, the Sri Lankan capital, and by Brig. Hamiliton Wannasinghe, the Army commander in the north, at a news conference in Jaffna, the main city in Sri Lanka's Northern Province.

Athulathmudali said all the rebel groups had not agreed to stop fighting and that the government might take action against those who kept up the battles, according to a United News of India report from Colombo.

But he said the government would proceed with projects on the Jaffna peninsula that had been suspended by the fighting.

The Indian report quoted Wannasinghe as saying that security checkpoints on the peninsula would be removed and fishing, which had been suspended, would be allowed within three miles of shore.