Despite congressional restrictions, the Reagan administration has continued secretly to assist anti-Sandinista rebels in finding weapons and plotting military strategy through a network of private operatives overseen by the National Security Council (NSC) and the CIA, administration and rebel officials say.

The system has been overseen by Marine Lt. Col. Oliver North, a senior staff member at the NSC, who has operated with advice from CIA officers attached to the Central American division of the agency's clandestine branch, the officials said.

The NSC and CIA involvement in contra activities comes in spite of an Oct. 10, 1984, law that forbids U.S officials from direct involvement in planning or executing military operations inside Nicaragua. The prohibition was relaxed -- but not lifted -- last summer when Congress authorized $27 million in so-called humanitarian aid to the rebels.

The officials said the system's main private agents include retired Maj. Gen. John Singlaub, the former commander of U.S. forces in South Korea, and Robert Owen, a 32-year-old Stanford graduate who until May 28 worked as a paid consultant for the State Department's Nicaraguan Humanitarian Assistance Office (NHAO).

PRIVATE CHANNELS USED

While any official military assistance to the contras would violate the law, the sources -- two administration officials and a knowledgeable rebel leader -- said the administration feels it has honored the restrictions by channeling its involvement through private citizens.

But the arrangement still is expected to be a central focus of congressional investigators looking into the contra movement. The investigation is expected to begin after the vote later this month on President Reagan's request for an additional $100 million in aid to the rebels.

North declined comment, but an administration official said North "has not been involved in illegal activities." CIA spokeswoman Kathy Pherson said the agency has "adhered" to congressional requirements.

The contra rebels were organized by the CIA and Argentine military officers in 1981 and began fighting the Sandinista government in Nicaragua in 1982 with CIA guidance and supplies.

In 1984, Congress banned CIA assistance for military actions inside Nicaragua after the CIA directed the mining of Nicaraguan harbors.

Under the ban, the CIA was allowed to receive intelligence from the contras but was barred from advising them.

CAN SHARE INTELLIGENCE

After Congress approved $27 million in U.S. aid to the contras last year, the CIA was allowed to share intelligence on Sandinista troop movements, but U.S. government officials were still prohibited from helping the contras plan strategy or procure weapons.

According to administration officials, North was first told to organize a network of private supporters for the contras in 1984, when it became apparent Congress was about to end CIA involvement in the rebel movement. It's not clear who in the administration told North to form the network.

Conservative activists were contacted and raised almost $30 million.

After Congress' ban prohibited North from dealing directly with the contras, private individuals were used as bridges between the administration and the rebels, officials said.

Singlaub, officials said, became the de facto military adviser to the contras while Owen served as a secret conduit for messages, advice and funds.