MANILA, Philippines (AP) _ Cardinal Jaime L. Sin told priests and nuns on Friday to avoid politics and Communist rebels after the military said it captured one priest among guerrillas and charged a second with arms violations.

The government's Philippine News Agency said in another development on Friday that the Foreign Office was investigating reports that the Soviet Union and Vietnam had offered to help the Communist rebel New People's Army here.

The rebels said the reports were part of a military ''disinformation campaign.''

Sin's directive reflected growing concern by the Roman Catholic Church over clerics in rebel ranks. It came in a set of guidelines for priests and nuns that Sin, the archbishop of Manila, released at a news conference.

The document followed years of warnings that circulated in the Philippines that Communists had infiltrated the church in this predominantly Christian nation of 56 million people.

The document, ''A Catechism on the Involvement of Priests in Political Activity,'' orders clergy not to run for public office or accept government positions, not to campaign for any candidate or political party in elections and not to join movements that espouse class struggle and violence.

Filipinos vote on May 11 to elect a national legislature and again on Aug. 23 for local and regional officials.

''Those people who advocate class struggle or violence as a means to change society do not work according to the mind of the church,'' said the Most Rev. Teodoro Bacani, one of three Manila-area bishops with Sin at the briefing. ''They do not have her support.''

Col. Renato Motus, a deputy regional police commander in the central Visayas islands, said Friday that weapons charges were filed against the Rev. Rogelio Arguelles after troops reported finding a hand grenade, ammunition and subversive literature in his rectory in rural Panay island.

The military announced Thursday it had captured a priest with a band of New People's Army rebels on Mindanao.

Despite the ban on political involvement by the clergy, Sin himself has made numerous statements strongly supporting the government of President Corazon Aquino.

Last year Sin called thousands into the streets to support military officers who deposed President Ferdinand E. Marcos. He said he was not motivated by politics but a desire to prevent bloodshed.

The Philippine News Agency quoted Foreign Secretary Salvador Laurel as saying he ordered a probe into a newspaper story that the Soviet Union and Vietnam had offered weapons, money and training to Filipino Communist rebels.

Efforts to contact officials for comment at the Soviet and Vietnamese embassies failed Friday night. But Soviet officials have said repeatedly they are not aiding local rebels and had no intention of doing so.

The New People's Army said the military had ''patently fabricated'' the reports and planted them in local newspapers. It also accused the military of exaggerating rebel casualty figures to impress ''their patrons in Washington.''

Also on Friday, former Defense Secretary Juan Ponce Enrile urged Filipinos to take a ''realistic'' view toward nuclear weapons believed to be stockpiled at the United States' Clark Air Base and Subic Naval Base.

He told reporters the Philippines should outlaw nuclear weapons only when it can afford to defend itself from foreign attack.

The new constitution says the Philippines ''adopts and pursues a policy of freedom from nuclear arms in its territory.''

The United States refuses comment about the presence of nuclear weapons on the bases or on ships that frequently call at Subic.

Mrs. Aquino fired Enrile after a purported coup attempt last November by officers close to him.