Saudi Oil Minister Ahmed Zaki Yamani told a conference in London yesterday that his country would maintain its oil production at 9.5 million barrels a day during this year's second quarter.

He added however, that Saudi Arabia would continue to produce at this level "only if we are satisfied we are getting a fair return on our investments abroad and help in developing our own alternative sources.'"

Saudi Arabia increased its output from a self-imposed ceiling of 8.5 million barrels a day July 1 in an effort to cool the spot market and promote a return to a unified pricing system by the Organization of Petroleum Exporting Countries.

At about $26 a barrel, Saudi Arabia's oil prices rank as the lowest in OPEC.

Addressing Britain's Institute of Directors, Yamani also warned that the world's oil reserves are being depleted "at an alarming rate" and called for an urgent international engergy program to "move our world away from the edge of an abyss.'"

U.S. government officials welcomed Yamani's statement on Saudi oil production, which has been a subject of concern since other producers have been cutting back their output lately.

"It's certainly beneficial to the West and industrialized countries," a State Department official said.

A spokesman for the Department of Energy said the Saudi decision was "kind of surprising" because there already is a slight surplus of crude on the world market despite the OPEC cutbacks.

Compared to last year, relatively few spot oil deals are being made now, and prices are running $3 to $5 above the average OPEC price of $29.06 a barrel. Previously, spot prices were running $8 to $10 above contract prices.

As the market stabilizes, the Saudis are more likely to go back to their "preferred production level" of 8.5 million barrels a day, analysts say.

"They will do it for sure," said an Arab oil specialist in Beirut. "The only question is when."

An Arab source who recently visited Saudi Arabia said he was told by Oil Ministry officials in Riyadh that a reduction would be necessary sometime this year.

The Saudis raised their production ceiling by 1 million barrels a day last year in response to a request by President Carter at a time when world supplies were particularly tight, sources said. Since then, the Saudis have reviewed the decision quarterly.

A recent declaration by Yamani that Saudi Arabia would not continue producing at current levels to enable the United States to build up reserves was taken as a sign that Saudi production will be allowed to drop when demand slackens further.

His statement came during a recent visit to the kingdom by U.S. Energy Secretary Charles Duncan for discussions concerning U.S. desires to resume buying oil for its strategic reserves.

The Carter administration has become increasingly eager to increase the reserves -- now standing at 92 million barrels -- since the Iranian and Afghan crises raised fears about possible interruption of supplies from the Persian Gulf. About one-third of U.S. oil imports, and even more of Western Europe's, come from the region.