Secretary of State Cyrus Vance called yesterday for the Organization of American States to mediate in the conflict between Nicaraguan President Anastasio Somoza and the Sandinista National Liberation Front.

At a press conference, Vance said the U.S. government has asked Somoza to "reflect carefully and deeply" on the civil war in his country.

"We have told President Somoza we believe that a political solution is necessary to resolve the problem," he said. Otherwise, "polarization will continue and the chances of a radical solution to the problem are great."

The Carter administration has made clear that it feels a takeover by the Sandinistas would be likely to result in domination by Marxist-Leninists in their ranks. At the same time, it has sought by diplomatic means to encourage Somoza to step down and has long since cut off aid to his government.

A previous diplomatic mission, headed by a State Department official but under OAS auspices, failed to convince Somoza that he should give way to his wide spectrum of political opponents. CAPTION: Picture, U.S. airmen and embassy personnel in Managua carry children and luggage to a waiting C130. The Americans are fleeing civil war violence in the capital. AP