The Reagan administration yesterday offered rewards of up to $100,000 for information leading to the "effective prosecution and punishment" of the gunmen who murdered six U.S. citizens at a cafe in El Salvador one month ago.

The offer came as officials repeated a formal U.S. warning to Nicaragua that the leftist Sandinista government is held indirectly responsible for those six deaths, and for any future "terrorist acts" against U.S. citizens anywhere in Central America.

State Department spokesman Robert Smalley said the reward offer does not apply to the Mideast, where hijackers killed one American and held 39 others hostage for 17 days last month, although the idea has been discussed.

"Each incident is considered on its own merits and this is a response to the incident in San Salvador," Smalley said at the regular news briefing. He called rewards "one step in a variety of methods that are available to us to pursue terrorists."

A branch of the leftist Revolutionary Workers Party of Central America (PRTC by its Spanish initials), one of the five members of the Salvadoran guerrilla coalition, claimed responsibility for the June 19 shooting in El Salvador that killed 13 people, including four U.S. Marines and two U.S. businessmen. The group called it an act of war against military allies of the Salvadoran government.

However, U.S. officials have consistently labeled such incidents terrorist acts and have said the Workers Party is linked to the Nicaraguan government. Smalley said that the U.S. warning to Nicaragua was based on "information we have available that . . . indicated a threat against specific U.S. targets.

"This was not a generalized terrorist threat," and therefore no travel warnings have been issued for Central America, he said.

President Reagan's national security affairs adviser, Robert C. McFarlane, told reporters separately that the charges against Nicaragua "derive from our knowledge, which is very concrete, that Nicaragua does support the PRTC, the group that has claimed credit and which evidence corroborates were involved, and so the connection is very clear."

However, he said, "If it were apparent that violence against Americans stemmed clearly from some other country, we wouldn't hold Nicaragua responsible for that. The reality is that the sources of terrorism in Central America are in very large measure supported by Nicaragua."

Nicaraguan officials vehemently denied the charges, accusing the United States of conducting its own terrorist campaign by supporting the contra rebels fighting Nicaragua.

The Salvadoran shootings occurred during the Mideast hostage situation, which led Reagan to say "our limits have been reached" in suffering terrorist attacks. Since then, the administration has drawn up plans, expected to go to Congress shortly, for a major program of antiterrorist training of police and military forces in friendly Central American nations. Congressional sources said they expect the funding request to be more than $50 million.

Rewards were authorized last year by the Act to Combat International Terrorism, which allows the secretary of state to give up to $500,000 to informants who enable terrorists to be brought to justice. Smalley called the act "a useful tool."

He and White House spokesman Larry Speakes said information received will be handled on a confidential basis and the identities of informants will be protected.