High-powered Cuban radio transmissions knocked two clear channel stations in Iowa and Utah almost entirely off the air for six hours Tuesday night, prompting the FCC to alert the State Department yesterday.

The interference, apparent retaliation for the Voice of America's Radio Marti broadcasts to Cuba, was aimed at WHO in Des Moines, the station where President Reagan once worked as a sportscaster, and KSL in Salt Lake City, a Federal Communications Commission spokesman said.

During the daytime on recent weekends, Cuba has interfered with WHO by transmitting Radio Moscow's English-language broadcast.

But Tuesday's interference on the frequencies used by WHO and KSL was the first time Cuba has struck so long at night -- when signals of the so-called AM clear channel stations can be heard hundreds of miles away, the agency said.

The Cuban relay of a mixture of English- and Spanish-language programming lasted from at least 6 p.m. until midnight EDT, FCC spokesman John Kamp said.

"We heard {it} at Grand Isle, Neb., at an FCC monitoring station," Kamp said. "It was also heard by an FCC staffer in Manassas, Va."

An FCC official said Cuba could be using as much as 300,000 watts of power. U.S. Class A clear channel stations operate at 50,000 watts.

Kamp said three stations have complained about stepped-up interference and that the FCC reported results of the monitoring to the State Department.

KSL assistant chief engineer Randy Finch said listeners could hear the station in the Salt Lake City area, but the signal was overpowered about 40 or 50 miles away. He said KSL went off the air twice during the evening for three minutes to help the FCC measure the strength of the Cuban transmissions.

Cuban radio interference has been a problem in southeastern states for 15 years. In 1984, when Radio Marti was preparing to go on the air, Cuba increased interference so much that Congress voted to compensate broadcasters forced to upgrade their towers to overcome the assault.