WASHINGTON--Former U.S. Ambassador Lewis A. Tambs testified yesterday that in 1985, while U.S. aid to Nicaraguan Contra rebels was banned, White House aide Oliver L. North ordered him to use his post in Costa Rica to help the rebels open a military front in Nicaragua.

"Mr. North was working for the National Security Council and obviously my assumption was that any instruction he gave me came from...his superiors," Tambs said in responding to a question at the Iran-Contra hearings about why he followed orders from the Marine lieutenant colonel.

Tambs also said he was asked by North to help private efforts to resupply the Contras.

Earlier in the day, former CIA operative Felix I. Rodriguez told the House-Senate hearings that he broke with the clandestine resupply operation for the rebels--run by North--after becoming disgusted with overcharges, bribery and deliveries of unreliable ammunition.

After Rodriguez completed his testimony, one of the legislators probing the affair, referred to the managers North recruited for the supply network as "thieving scoundrels," adding to the debate over whether those aiding the rebels were patriots or profiteers.

Tambs, who was ambassador to Costa Rica from 1985 until last January and is now a history professor at Arizona State University, gave the committees the first direct evidence of a State Department official helping the rebels during the two-year congressional ban on U.S. military aid to the Contras.

He said he did not question the legality of his instructions, which came from a small group that included North, then a member of the National Security Council staff, the head of the CIA's Central American task force and Elliott Abrams, who was awaiting Senate confirmation as assistant secretary of state for Central American affairs.

When pressed on the point, Tambs said, "They have a saying in the Foreign Service: When you take the king's shilling you do the king's bidding."

He dismissed questions about whether he should have sought a legal opinion from the State Department or White House about undertaking the operation.

"You can't really expect people in the field to be constitutional lawyers.... The people in the field who are trying to do a job are going...to assume that orders from Washington are legal and legitimate," Tambs said.

Nonetheless, Tambs said he avoided official contacts with Contra leaders because of the congressional ban. Instead, he said, "I aided the private, patriotic Americans" who were supporting the Contras.

Just before assuming his post in Costa Rica, Tambs said, he and North met in North's office. At that meeting, "Colonel North asked me to go down and open up the southern front" in neighboring Nicaragua for the rebels, Tambs said.

Tambs also testified that he negotiated with Costa Rican officials for construction of the Santa Elena airstrip in the northern part of the country for use by North's private airlift supporting the Nicaraguan rebels.

The former ambassador said that Abrams, the late CIA Director William J. Casey and former National Security Adviser John Poindexter all discussed the airstrip with him.

Secretary of State George P. Shultz refused to discuss Tambs' testimony at a news conference yesterday. Shultz said he would say what he had to say when he appears as a congressional witness.