A Roman Catholic priest and a Seventh Day Adventist minister have been taken prisoner in the Nicaraguan countryside by a contra guerrilla group that allegedly threatened to execute them at dawn today, according to fragmentary reports from Nicaragua.

Information received by several religious and human-rights organizations in this country indicated that Father Enrique Blandon and the Rev. Gustavo Adolfo Tiffer were seized about a week ago in the central province of Matagalpa.

Several of the groups also said they were told that a clandestine contra radio, Radio Liberation, had announced in a broadcast yesterday morning that the two would be executed at 6 a.m. today. However, Bosco Matamoros, a contra spokesman here, said he had checked with contra leaders in Central America and could state categorically that no contra-controlled radio had made such a "monstrous suggestion."

He also said the contra leadership had no information about the alleged prisoners. He added: "If they have been detained by our forces, they are not considered by us to be prisoners of war, and they will be released."

State Department officials said last night they were looking into the situation but had nothing to report. In Nicaragua, Bishop Salvator Schlaefer, the Catholic bishop of the Bluefields diocese, issued a statement expressing anxiety about the safety of the two men and calling for their release.

If they should be harmed, it would be a severe blow to the efforts of the contras and their backers in the Reagan administration to persuade Congress to approve $270 million in new military aid to continue the resistance to Nicaragua's Marxist Sandinista government. On Thursday, the contras' chief political leaders announced here that they soon will send a delegation to Managua in an effort to force the Sandinistas to negotiate with them as part of the five-nation Central American peace process.

According to information reaching here, the most extensive report about the two clergymen appeared yesterday in Barricada, the official Sandinista newspaper. It said Blandon and Tiffer are both residents of Waslala and members of a regional peace commission set up by the Nicaraguan government to discuss cease-fire and amnesty questions with contra guerrillas in the field. The government has said it will not talk with their political leaders outside the country.

On Oct. 10, the Barricada report continued, the two left Waslala in response to a request from a Commandante Danto, allegedly the commander of the Santiago Meza brigade, for discussions. Instead, the report said, they were taken prisoner, along with two Catholic nuns who later were released.

A communique issued Tuesday in the name of Enrique Bermudez, the contras' military commander, said Blandon had been detained briefly and then released. But yesterday's statement by Bishop Schlaefer, who has gained international renown for his human-rights efforts in the Nicaraguan civil war, indicated both men were still missing and said church authorities were very concerned about the death threat.

Matamoros said the contra leadership regards the peace commissions as a Sandinista trick to avoid real negotiations and has warned that any commission members who come into guerrilla areas will be detained and sent back. But, he added, any suggestion that such detainees would be mistreated or held indefinitely was contrary to contra policy, and he suggested the incident might be a Sandinista ploy to influence U.S. public opinion.