The World Bank is sending an economic mission to Hanoi later this month in the first step toward an internationally financed program to help reconstruct war-torn Vietnam.

The five-member mission headed by Edward Hawkins, one of the bank's senior economists and a British citizen, is scheduled to arrive in Hanoi, via Peking, on Jan. 18 for a month-long study of the country's economic condition, policies and needs.The dispatch of the study mission, which was discussed with Vietnamese officials last October, did not require approval by the United States. World Bank officials said the United States knew about the mission but lodged no objections.

The United States would have a vote an any eventual loans from the World Bank for Vietnam, but would be unable to block them without votes of other member nations even if the incoming Carter administration should disapprove. There is no clear indication of President-elect Jimmy Carter's views on the subject.

In a related move, the Asian Development Bank - of which the United States is also an important member - dispatched a seven-member mission last Sunday to study the Hanoi government's request to take over 11 loans which had been granted to the former Saigon regime.

The loans, totaling more than $44 million, were to finance improved water supply and telephone systems for Saigon, a fishing industry, agricultural development in the Mekong Delta and an industrial development program. Only a small amount was disbursed before Saigon fell in April, 1975.

Last September the Hanoi-based Socialist Republic of Vietnam was granted permission to take the former Saigon government's place among the members of the World Bank, International Monetary Fund and Asian Development Bank, despite U.S. objections. Vietnamese officials made it immediately clear they hope for major international assistance in the formidaable task of repairing the war damage.

Tran Duong, vice minister of Hanoi's State Bank of Vietnam, told the annual meeting of World Bank and International Monetary Fund in Manila last October that his country deserves "the special concern" of the two international institutions because of its relatively low income and severe devastation by war.

Bernard R. Bell, World Bank regional vice president for East Asia and Pacific, said in an interview that Duong told him at the time that the war damage was more serious in the north than in the south, and had been far more costly than publicly admitted during the years of fighting.

Victor Umbricht, a Swiss diplomat and industrialist who went to Vietnam on a United Nations mission last year, reported that $432 million is needed just to rehabilitae the transportation system and recultivate abandoned land, Umbricht reported that "some villages have been totally erased from the earth - you have some cities without a house left standing."

President Nixon promised $3.25 billion in postwar reconstruction aid to North Vietnam in early 1973 in connection with the Paris accords, which were intended to end the fighting. But the pledge was renounced after the fighting flared anew, and U.S. officials have said that in any case it was subject to approval by congress.

The United States has twice voted the admission of Communist Vietnam to the United Nations, most recently last November, on grounds that Hanoi has failed to account for Americans missing in action. Hanoi in turn has linked further information with the promised U.S. reconstruction program.

here has been speculation that the United States may be willing to finance a Vietnamese reconstruction program indirectly through the World Bank and other such agencies even while balking at direct contributions. Vietnam is receiving United Nations aid, mostly for humanitarian purposes, without U.S. objections.

A House committee on Americans missing in action urged last month that the United States consider supplying "humanitarian aid, but not war reparations."

The committee estimated that $34 million in U.N. loans and grants to Vietnam this year originated in U.S. contributions.