A top State Department official indicated yesterday that the United States is willing to have "informal contacts" with rebels fighting the Marxist government in Mozambique but will not extend recognition and support to them as Washington has to rebels in Marxist Angola.

Defending the U.S. policy of strong backing for the Mozambican government, Assistant Secretary of State Chester A. Crocker said Mozambique's recent turn toward the West and decision to establish closer ties with Washington had dealt the Soviet Union a "severe blow" in southern Africa.

Crocker called the U.S. policy, under attack from conservatives led by Senate Minority Leader Robert J. Dole (R-Kan.) and Sen. Jesse A. Helms (R-N.C.), "a case study of success in action on the ground" in weaning a pro-Soviet Third World nation away from once-close association with Moscow.

But an angry Helms charged that Crocker's testimony contained "astounding contradictions, adding, "You don't know what you're talking about, or otherwise you have come up here to defend an indefensible policy of the U.S. State Department."

The senator also attacked the State Department's policy of refusing to hold talks with the Mozambican National Resistance (RENAMO), which is leading the widespread antigovernment insurgency in Mozambique, while Secretary of State George P. Shultz had agreed to meet publicly with leaders of the African National Congress (ANC), the leading black nationalist opponent of South Africa's white minority government.

Helms said he wants the Reagan administration to treat equally "all freedom fighters" around the world. If the United States was willing to deal with the ANC and provide military support to anticommunist rebels in Angola, then it should also talk to RENAMO, Helms said.

"All the State Department has to do is say we are going to do the same thing with RENAMO that we do with the ANC," Helms told Crocker. "But, no, the State Department is stonewalling."

Crocker said he agreed with Sen. Paul Simon (D-Ill.), Senate Africa subcommittee chairman, who suggested the United States ought to maintain "some kind of informal contacts" with RENAMO to help facilitate a political settlement to the conflict with the central government.

But Crocker said the administration did not want its officials or diplomats to be used "for a photo opportunity" by RENAMO to give the insurgency international legitimacy when it lacked what he called "a credible political identity" inside Mozambique itself.

He also argued that U.S. recognition of RENAMO would isolate the United States from its Western allies, including Britain, which has just increased military and economic aid to Mozambique, as well as from the other black southern African states, which he said support the central Mozambican government against the insurgents.

Crocker said recognition of RENAMO would imply U.S. endorsement of South Africa's "destabilization efforts" towards its black African neighbors. "There is credible evidence that South Africa remains a reliable supplier of high-priority {military} items that RENAMO is not able to acquire on its own," he said.

South Africa and RENAMO deny that the South Africans are supporting the insurgency, although Crocker has said repeatedly that the State Department believes Pretoria is.