The Rhodesian military has succeeded in smuggling up to 11 U.S. designed Huey helicopters through international arms and economic embargoes and has put them to use in the escalating war against African guerrilla forces, according to authoritative diplomatic reports.

The bell 205A s spotted by visitors to Rhodesia in the past three weeks are a civilan verson of the UH1 Huey Iroquois, the U.S. workhorse for ferrying tropps in the Vietnam war. They significantly expand Rhodesian air strike capability.

U.S. officials said yesterday a formal investigation has been launche by the State and Commerce departments to determine how the helicopters reached Rhodesia, which has been under international trade embargo since the shite settler government unilaterally declared independence from Britain in 1965.

In the search for intermediaries, countries that were targeted for initial investigation included South Africa, Iran and Israel, these officials said. But a check of recent sales records produced no evidence that these countries were involved, and the search is now focusing on private companies in the United States or abroad that might have purchased civilian models of the Iroquois and sold them illegally to Rhodesia.

Rhodesia appears to have converted the 205A to military use by adding extra armor plate and modifying it for machine guns on the side doors as was done on U.S. helicopters in Vietnam, according to witnesses.

By one account, the particular model now in Rhodesia appears to resemble the Agusta Bell 205A, which is manufactured in Italy under the license from Bel Helicopter.

In response to a question, State Department spokeswoman Mary Ann Bader said the Unted States is investigating reports that 11 Hueys have turned up in Rhodesia in recent weeks. Formal approval is required for the sale or transfer of the helicopters, and no such approval has been granted for Rhodesia, she said.

A western intelligence source reports said 25 Italian-made Hueys were sold to South Africa in 1974 or 1975. For South Africa to shift Hueys to Rhodesia would be a major upgrading of the supplies Pretoria is sending to Prime Minister Ian Smith's government, a U.S. official said. This official was skeptical that the South Africans would part with an aircraft that has now become difficult to obtain.

Rhodesia appears to be significally expanding its helicopter force to combat the hit-and-run war waged by the Patriotic Front forces of Joshua Nkomo and Robert Mugbe. The London-based Institute for Strategic Studies, in its authoritative annual Military Balance report, shows the number of French-designed Alouette helicopters in Rhodesian hands jumping from 16 in 1976 to 66 this year.

South Africa's military is a principal purchaser of the troop-carrying Alouette.

Disclosure of the Hueys in Rhodesia comes at a particularly sensitive moment for U.S.% DIPLOMATIC EFFORTS. RPIME MINISTER SMITH'S VISIT TO THE UNITED STATES IN OCTOBER HAS SPREAD DOUBTS AMONG THE AFRICAN NATIOALISTS AND THEIR SUPPORTERS ABOUT U.S. intentions. Suggestions of U.S. complicity in supplying helicopters or other weapons systems to Rhodesia would severely erode African confidence in the Carter Administration.

U.S. officials confirmed that the State and Commerce departments are also investigating reports that roughly 20 Cessna FT337B reconnaissance planes manufactured in France under U.S. license also have turned up in Rhodesia.

Rhodesian airborne and bombing raids against guerrilla camps in Zambia and Mozambique in recent months have, according to some diplomatic sources, won some time for Salisbury in warding off a major guerrilla invasion.

Assistant Secretary of State for African Affairs Richard Moose told reporters at a luncheon yesterday that the situation in Rhodesia is "getting progressively worse," that violence is increasing on all sides.

Moose voiced optimism that South Africa will soon agree to the introduction of United Nations troops and new elections in the disputed territory of Nambia, where a separate black vs.

white Guerrilla war is being fought.

The United States has considered but not "dwelt" on the possibility of endorsing economic sanctions if South Africa finally rejects the U.N. plan, Moose confirmed, but he added that South Africa has shown flexibility without being threatened with sanctions.