Sandinista military officials claim that 35 civilians have been killed by mines they say were planted in roads in northern Nicaragua by the anti-Sandinista guerrillas as a new tactic in a spring offensive.

A leader of the Nicaraguan rebels this week denied the charge, saying the counterrevolutionaries, known as contras, do not have access to mines. But a knowledgeable foreign source said last week that the contras began to get mines several months ago. Nicaraguan and foreign sources provided information confirming that 10 of the victims were killed by explosives set into roads.

"They contras are using more mines," the knowledgeable foreign source said.

Reports of stepped-up mining coincide with an offensive begun this spring that has brought fresh fighting and sabotage by the U.S.-backed rebels after a lull of more than a year. The offensive, according to a U.S. State Department official and diplomatic sources interviewed last month, was made possible primarily by the arrival of supplies such as boots bought with $27 million in congressionally approved humanitarian aid that had been blocked by Honduran officials from delivery to the contra camps until last winter.

These sources did not link the mines to the supplies purchased with the $27 million in aid, and it was not possible to establish how the mines were obtained by the contras or what financing was used.

Nine civilians, including a Spanish medical worker, were killed when their truck ran over a mine near San Jose de Bocay in late May, according to an official at a European embassy.

A Nicaraguan friend of the medical worker, who went to the town, said Monday that he saw the Toyota truck "in pieces" and a hole in the dirt road three feet in diameter. He said the nurse who prepared the body told him some of the bodies of the agricultural, health and education workers were blown apart.

In the provincial capital of Jinotega, coffee farmer Enrique Casco last week said his wife, who was buried last Wednesday, was killed and her driver wounded when their truck ran over a mine near his ranch near the town of Pantasma.

A military spokesman for Matagalpa and Jinotega provinces said in an interview here last week that 26 civilians were killed by mines during the past two weeks of May.

A military officer in Jinotega said another nine persons were killed in the first two weeks of June. He showed reporters mines he said were recovered by Sandinista soldiers.

Adolfo Calero, leader of the contra umbrella organization, the Unified Nicaraguan Opposition, said the Sandinistas may be planting mines in their own territory to discredit the contras. "They [Sandinistas] are the ones who are known to have used them [mines]," he said in a telephone interview from Miami on Monday.

The Sandinistas reportedly have used antipersonnel mines in the rugged montain terrain on or close to the border to keep Honduras-based contras out of Nicaragua.

The deaths of thenine health, education and agricultural workers and the Jinotega woman occurred along the roads as far as 35 miles from the border. Contras, who are known to travel on foot through mountain terrain rather than by vehicle on roads, are not likely to fall victim to mines planted on those roads that are used by Sandinista military vehicles.

Analysts say although mining can draw charges of human rights abuse, it would allow guerrillas to demonstrate a presence without losing fighters.