A national security aide to Vice President Bush during the Iran-contra scandal has testified before a federal grand jury conducting a criminal investigation of the affair.

G. Philip Hughes, who has been nominated by President Bush to be U.S. ambassador to Barbados, declined to discuss his testimony after being questioned Wednesday for nearly two hours by Iran-contra prosecutors.

Until recently, Hughes was the National Security Council's executive secretary. He was deputy assistant for national security affairs to Vice President Bush from 1981 to 1985, and National Security Council director for Latin American affairs in 1985 and 1986.

Because of a political dispute unrelated to the Iran-contra affair, the Senate Foreign Relations Committee has not acted on Hughes's ambassadorial nomination, and it was returned to the White House this week. The White House can resubmit the nomination when Congress reconvenes in September.

Hughes is thought likely to have testified to the grand jury about his work with Iran-contra figures Oliver L. North and Donald P. Gregg in the period 1985-86, when Congress had cut off U.S. military aid to the Nicaraguan contra rebels. Gregg, a former CIA officer, was Bush's top national security aide when he was vice president and now is U.S. ambassador to South Korea. North, then on the National Security Council staff, was the overseer of a secret contra resupply network during the cutoff period.

Gregg has said he was unaware until August 1986 that North was involved with contra resupply, and did not know the full extent of North's role until December of that year. Bush has said he was unaware of North's network until it was publicly exposed in December 1986.

In 1985 and 1986, Hughes met periodically with North. His name turned up as early as July 1984 in the 2,600 pages of North's notebooks that were released last spring.

Hughes was copied in on a September 1984 memo to Bush from Gregg about private financing for the contras just before the congressional ban on U.S. military aid to the rebel force went into effect.

Hughes also was involved in a 1985 incident in which Bush signed a letter suggesting that a Guatemalan physician interested in supplying medical aid to the Nicaraguan rebels contact North.