Bangladesh President Ziaur Rahman met with President Carter yesterday to thank the United States for past assistance and explain Bangladesh's need for even more aid in the future.

Zia told a press conference that the White House meeting "was extremely useful to us." Bangladesh, one of the world's poorest and most densely populated nations, with more than 80 million people in an area the size of Florida, is embarking on a new five-year plan that will cost $18 billion at present price levels.

Bangladesh requested $2.8 billion in U.S. assistance this year and has received commitments of $1.5 billion. Aid last year was $84 million.

Among the goals of the five-year plan, the 43-year-old president said, is a doubling of agricultural output. "Our land has the capacity, but it is badly organized," he said. Planned agrarian reforms will get greater production from the land, he added.

Zia came to Washington for a few hours yesterday after addressing the U.N. General Assembly Tuesday, where he called for greater investment in developing nations by the Organization of Petroleum Exporting Countries. He said he had received initial favorable reaction from some OPEC members, but he declined to name them.

"The world has to look at this problem collectively," the former army officer said of the vast transfer of resources resulting from the rapid increases in oil prices.

On foreign policy, Zia was asked whether he approved of the expanding U.S. military presence in the Indian Ocean and Persian Gulf regions. "That's a difficult question to answer," he replied.

He said that he and Carter discussed the Iranian hostage crisis in only a general way.

Zia reiterated to Carter his nation's stand that Soviet troops should withdraw from Afghanistan.

Although Bangladesh's relations from India have been strained by disputes over rights to water in the Ganges River system and about their border, Zia said that new talks are scheduled and he hopes for a new understanding with the giant neighbor whose army was instrumental in winning Bangladesh's independence from Pakistan in 1971.

Zia left Washington yesterday for New York, Paris and home without a specific commitment of new support from the United States, but he said, "President Carter assured me of all possible assistance, especially the agricultural sector."