Belgian Prime Minister Wilfried Martens told President Reagan yesterday that he still supports the North Atlantic Treaty Organization decision to base U.S.-made, medium-range missiles in Western Europe, but he refused to say whether Belgium will begin deploying its share of the missiles in March as planned.

Nevertheless, U.S. officials said later that the White House meeting had left them "optimistic" that Martens' center-right government eventually will go ahead with deployment of the 48 cruise missiles despite strong pressure from antinuclear groups and opposition parties.

Reports last night from Brussels quoted Belgian television as saying that Martens' cabinet intends to approve deployment "in principle" in March, but will hold further consultations about a timetable for actual placement of the missiles.

U.S. and Belgian officials here said they could not confirm that report. A senior U.S. official, who briefed reporters after the White House meeting on condition that he not be identified, replied to questions about whether deployment will begin in March by saying, "I would not rule anything out."

Martens later told reporters that the final decision may not come until late March and indicated that he would take the matter to parliament.

To support their expressions of optimism, the officials cited Martens' public comment to Reagan as he left the White House. He said:

"I reaffirm our attachment to the NATO dual-track decision, which is an expression of firmness in defense and of openness for dialogue."

That was a reference to the 1979 NATO decision to station 572 cruise and Pershing II missiles capable of striking deep into the Soviet Union in five NATO countries unless the Soviets agree to reduce substantially the number of their medium-range missiles in Eastern Europe.

The U.S. officials, anxious not to be perceived as interfering in Belgium's internal affairs, were cautiously circumlocutory in discussing the talks between Reagan and Martens. But they left the impression that Martens is trying to use the time between now and March to buttress his position by collecting expressions of public support for deployment from Reagan and leaders of other NATO countries.

"The Belgian government is beginning a process of consultation with the United States and other NATO countries before making a specific decision on deployment, and we think the process got off to a very good start today," the senior U.S. official said. "After this first consultative stage, we are very reassured."

Belgium's adherence to the 1979 NATO agreement is regarded as very important by the Reagan administration, which believes that NATO must continue to show solidarity on the missile question to strengthen the U.S. bargaining position in the new arms control talks agreed to by the United States and the Soviet Union in Geneva last week.

Medium-range missiles are one of the three categories of weapons, along with strategic missiles and space weapons, to be dealt with in the negotiations.

Three NATO countries -- Britain, West Germany and Italy -- already have started positioning their share of the missiles. But progress has been stalled in Belgium and the Netherlands, where fierce antinuclear sentiment has made deployment an issue of such controversy that it has threatened to topple the governments in both countries.

After repeated delays, the Belgian government indicated last November that it would begin deployment this March. But Martens then found himself faced with a potential revolt from powerful forces in his Christian Democratic Party, who argued that Belgium first should see what progress resulted from new U.S.-Soviet talks.

That forced Martens to say he would defer a decision until after conferring with Reagan and other NATO leaders.

U.S. officials said that in yesterday's meeting, Reagan argued that NATO's past solidarity on the deployment issue had been a crucial factor in forcing the Soviets back to the bargaining table after they walked out of arms-control talks last year. The president reportedly added that if Belgium delays its deployment commitment, it would have a "counterproductive effect" on the new arms talks.

According to the officials, the two leaders also agreed on closer consultation among NATO members to avoid disagreements like those that broke out between the two countries last year over U.S. opposition to Belgium's desire to sell high-technology equipment to several communist countries.

Regarding a shelved Belgian plan to sell nuclear power facilities to Libya, the senior official said that "the view of the U.S. government is very, very negative."