Brussels — The talk at NATO may have turned to improving the West's conventional defense. But the action - for now at least - has returned to nuclear weapons. Last week, Belgian Prime Minister Wilfried Martens's Christian People's Party voted to delay deployment of 48 new US nuclear missiles in light of the arms talks scheduled between US Secretary of State George Shultz and Soviet Foreign Minister Andrei Gromyko in Geneva next month.
''We must give the new negotiations a chance,'' party president Frank Swaelen said.
Officials note that next March Belgium is due to begin deploying its share of 572 new nuclear missiles scheduled to be stationed in five West European countries under a 1979 NATO plan. So far, however, the Belgian government has refused to give the green light to deployment, saying it will do so only if the US-Soviet arms talks fail. West Germany, the United Kingdom, and Italy have begun to deploy their share of the new weapons. But neither Belgium nor the Netherlands has given the nod.
Last June, under heavy pressure from the country's antinuclear movement, the Dutch government agreed to delay its final decision on deployment until next November. If at that time the Soviets have deployed more than 378 SS-20 missiles (the number announced by NATO last December), deployment of 48 US cruise nuclear missiles in the Netherlands would begin on schedule, the government said.
Earlier this week, the US State Department gave the Dutch government the news it may have wanted to hear - although many Dutch citizens certainly did not. The US said it had evidence that the Soviets had now deployed 387 SS-20s. The news was relayed to the Dutch government - and to the other countries of the Western alliance - by US Defense Secretary Caspar Weinberger at a meeting of NATO defense ministers this week in Brussels.
But Dutch government leaders said that announcement would not change anything for the time being. ''We'll still be looking to see how many missiles (the SS-20 s) the Soviets have deployed on Nov. 1, 1985,'' a senior Dutch official said.
US sources said Mr. Weinberger told his NATO colleagues here this week that the situation had been complicated by an ambitious base reconversion program undertaken by the Soviets in recent months, making accurate counting of SS-20s in ''deployment status'' extremely difficult. ''The exact number varies from day to day,'' an official said.
NATO officials point out that the Soviets would not have to be too clever to temporarily reduce the number of SS-20s in ''deployment status'' just prior to the Nov. 1, 1985, Dutch deadline.
Meanwhile, the situation in Belgium has become more complex.
Until last week, Mr. Martens's center-right coalition government had been widely expected to approve deployment. Now there is some doubt not only about that but also about whether the government will survive.
The leader of Martens's party in the Parliament has said that if the government approves deployment before next March, his group will withdraw its support for the ruling coalition. But Vice-Premier Jean Gol, who belongs to the other main party of the four-party coalition, has said that if the US-Soviet talks have ''led to nothing'' by next March, ''it will be necessary to implement our part of the common defense effort.''
The apparent willingness of the Soviets to return to the negotiating table has changed the facts, Mr. Gol said. ''Not a single SS-20 has been dismantled, on the contrary.''
Martens has been careful to stand aloof from the growing split within his 15 -member Cabinet over the issue. Asked by the Monitor whether he supported his party's decision to delay deployment, he replied, ''No comment. I represent the government.''
The government's position - reaffirmed last week - is that it will take a decision to deploy or not after evaluating the climate of East-West relations and the now-nonexistent US-Soviet arms talks on limiting intermediate-range nuclear missiles in Europe.
Next month could be crucial. Martens pays an official four-day visit to Washington. President Ronald Reagan is certain to raise the missile issue - and probably urge the Belgians to begin deployment on schedule.
''It will be an important visit,'' Martens said. ''And it will be a key element in our evaluation of the international situation.''