Less than a year after the State Department spent at least $140,000 to provide country-coded, red, white and blue license plates for the cars of foreign diplomats, a handful of missions -- including the Soviet Union, South Africa and Iraq -- have asked for and received a second new set of tags.

A State Department official said the Soviets asked for the change last summer, soon after The Washington Post published a list of the codes identifying 18 countries on a special watch list used by FBI counterintelligence agents to keep tabs on suspected spies.

"They complained about vandalism, though we couldn't verify any cases," said the official, who asked not to be identified.

So starting two weeks ago, the plates of Soviet cars at its embassy here, as well as its office at the United Nations in New York and its consulate in San Francisco, were changed so the country code -- a two-letter prefix on the side of the tag -- now reads FC, rather than SX. The change is easy to spot by reading the plates of the cars parked along 16th Street in front of the Soviet Embassy.

The switchover for more than 200 Soviet cars cost the State Department at least $2,000, the official said. The tags are made at the District of Columbia's Lorton reformatory.

Boris Malakhov, press attache at the Soviet Embassy, said he did not want to comment on why his country's diplomats requested a new designation. "I don't think that's a matter of vital importance to your newspaper," he said. The cars of U.S. diplomats in the Soviet Union have carried the designation "04," identifying them as American, for years.

Three other countries, including two on the "criteria list" of nations the Federal Bureau of Investigation watches for possible spies, also have had their tags changed, the State Department official added. He declined to say which ones they are.

Pieter Swanepoel, media spokesman for the South African Embassy, which has been the target of demonstrations by protesters of the country's apartheid policies, acknowledged that his country had requested a change, but declined to say why. "We took it up with the State Department quite some time ago, but I'd rather not comment on the exact reasons," he said. Cars leaving the South African Embassy's underground garage on Massachussetts Avenue NW this week bore tags beginning with BL. Formerly, they were designated FY.

Sources said South Africa is on the watch list because its intelligence service shadows U.S. diplomats posted there and because its diplomats are suspected of trying to gain access to American technological secrets.

Iraq, which is on the list because of its alleged support of terrorist activity, has had its code changed too, from TS to BZ. Cars parked at the Iraqi Embassy on 18th Street NW all had BZ plates yesterday. A spokesman for the embassy did not return a reporter's telephone calls.

The State Department official said he was concerned that continued publicity about the country designators could invite harassment of U.S. diplomats abroad or pressure from foreign embassies to have the new system scrapped. "And with Gramm-Rudman, we can't afford to keep changing them either," he said with a laugh.

The State Department official also acknowledged that it would not be difficult for any interested person to find out, by simple observation, what a particular country's license plate code is. "I'm not making the claim it's a national security secret," the official said.

Another government source noted that many FBI agents would welcome publicity about the Soviets' new FC designator in the hope that interested citizens would notify the bureau if they spotted Soviet officials driving in deserted rural spots outside Washington.

Soviet diplomats are limited to traveling within a 25-mile radius from downtown Washington unless they have permission from the State Department. Those traveling far outside the city limits of Washington might be up to such intelligence work as scouting for or emptying a "dead drop," where spies hide documents and photos. For instance, alleged Soviet spy John Walker was arrested last summer near Poolesville, Md., at the edge of the 25-mile zone. A Soviet official was seen in the area the same day and later left the country