Cuban MiG fighters streaking over the Straits of Florida on Saturday shot down two single-engine planes belonging to a refugee relief group, U.S. officials said. American warships steamed close to Cuban waters Saturday night.

Four crewmen were missing and presumed dead. Coast Guard and Navy planes and helicopters searched into the night, but found only oil slicks. President Clinton said he instructed U.S. military forces to support the rescue operation “to ensure that it is fully protected.”

U.S. officials told The Herald that the planes apparently were shot down over international waters but in an area under the jurisdiction of Cuban air traffic controllers.

A third plane returned safely to South Florida. Aboard that plane was Jose Basulto, who created Miami-based Brothers to the Rescue in 1991 and has been its most visible representative.

Spokesmen for the Brothers said the downed planes were looking for wayward Cuban rafters and never violated Cuban airspace; U.S. officials said they were not certain of that.

“At no time did they veer into Cuban airspace,” said Guillermo Lares, a Brothers pilot.

“It was a search-and-rescue mission. We are asking the exile community to pray for our pilots.”

He identified the missing men as Carlos Costa and Pablo Morales on one plane and Mario de la Pena and Armando Alajandre on the other plane.

But a Pentagon official told The Associated Press that early indications suggested the planes may have been heading to Cuba to pick up people and fly them out of the country.

White House Press Secretary Mike McCurry said F-15 fighter planes were scrambled to provide air cover for the rescue operation, which was centered about 15 miles northwest of Havana, close to Cuba’s 12-mile limit.

President Clinton, addressing the nation Saturday night, said he demanded an explanation of the incident from Cuban officials.

“I condemn this action in the strongest possible terms,” he said.

Campaigning for the Republican presidential nomination in Arizona, Senate Majority Leader Bob Dole said, “I don’t know particulars but now is not the time to ease up on Castro as the administration may be trying to do.”

Top administration policymakers from the National Security Council, State Department and other agencies were to meet this morning to discuss the matter.

McCurry said three small planes had taken off from Florida and filed flight plans saying their destination was the Bahamas. He said they apparently were en route to Cuba.

“They apparently were engaged near the territorial waters” of Cuba, McCurry said.

He said U.S. officials had been unaware of the real destination of the planes.

“They clearly had detoured” from their flight plan “if they were in this vicinity,” McCurry said. “We would not have accepted flight plans indicating Cuba as a destination.”

The Cuban-American community in Dade County and throughout the nation mobilized for protests - and for prayers. Supporters waving Cuban flags descended on the Brothers’ hangar at Opa-locka Airport. Dozens of Metro-Dade police officers patrolled the area.

“What we feel is indignation,” said Armando Perez, one of many people who gathered in Miami’s Little Havana to monitor reports of the incident.

“The planes weren’t going to attack Cuba. They didn’t have arms. It was very dangerous, but they just went there to give the Cubans some hope.

“The only thing we can do is to cry. That’s the only thing we can do.”

The search area was centered in the Straits of Florida, about three miles north of the 12-mile limit claimed by Cuba, according to the Coast Guard.

Petty Officer Scott Carr said two oil slicks, possibly from the downed planes, were spotted in the area before darkness descended. He said the first report of the downing came to the Coast Guard at 3:45 p.m. from the Federal Aviation Administration.

The U.S. Navy dispatched all available rescue ships and aircraft to the area.

“U.S. Atlantic Command Forces are responding to the reported downing of two civilian aircraft by Cuban fighters,” said Navy Capt. Craig Quigley, with the U.S. Atlantic Command in Norfolk, Va.

Air Force reserve pilots were summoned to fighter squadrons at the Homestead Air Reserve Base, The Herald learned.

There was no immediate statement from Cuban officials in Havana or at the Cuban Interests Section in Washington. Cuban citizens with access to satellite television services first learned about the incident from American news programs.

The Cuban exile group has specialized in humanitarian missions but recently turned provocative with two overflights of Havana since July. Cuban President Fidel Castro warned that any aircraft violating Cuban airspace risked being destroyed.

Still, the Brothers’ missions were almost universally regarded as humanitarian. The one exception - the Cuban government.

According to George Dorrbecker, president of the Cuban American Pilots Association, who has flown missions with Brothers pilots, local FAA authorities started issuing new advisories on Cuba about three weeks ago.

“All pilots were told that if you cross the 24th parallel without a flight plan, the Cuban government would not be responsible for your personal security,” he said. “They said they had military operations in the area and antiaircraft guns were operative.”

American officials said the Cubans radioed the exile planes and said: “The area north of Havana is active and dangerous and you must remain north of the 24th parallel.”

There was no indication the Brothers planes responded to the Cuban warning, according to U.S. officials.

The 24th parallel cuts through the Florida Straits approximately half way between Cuba and Key West. According to Dorrbecker, Cuba has international policing authority beyond that point and a Cuban air traffic dispatcher takes control of all flights.

“That means if you enter there without permission, they can come out and identify you,” he said. “Maybe they will waggle their wings, ask you to identify yourself, maybe even make you follow them and land in Cuba.

“But to shoot them down, that is a major transgression of international law. They were on a peaceful mission. They were unarmed.”