Worldwide concern sharpens on issue of nuclear proliferation.

The Belgian government has begun discussions with its European allies on whether to accept a $1 billion offer from Libyan leader Muammar Qaddafi to help his country build its first nuclear power plant.

For months the Reagan administration has pressured Belgium to reject the offer, arguing that the technological knowledge Libya would gain from the deal could help Colonel Qaddafi acquire a nuclear bomb.

If Belgium balks, the contracts could go to other West European countries, principally France and West Germany. The Belgian government wants some assurance from them that they won't grab the business if Belgium withdraws.

Noting that Belgium faced ''an extremely difficult and delicate'' situation, Economics Minister Marc Eyskens told parliament last week that the government still had not decided whether to approve the deal. Foreign Minister Leo Tindemans was consulting other European countries to reach a ''consensus,'' he said.

Only Britain, which has had its problems with Libya in recent months, including the siege at the Libyan Embassy in London last spring, appears ready to give Belgium the assurances it wants.

At issue is a standard-size nuclear power plant which Libya hopes to begin building on the Gulf of Sidra in June 1986. The Russians have promised to supply Libya with the nuclear heart of the reactor, including the fuel. Qaddafi has given Belgium, a world leader in nuclear technology, first shot at the nonnuclear contracts for the plant.

For a fee of about $33 million the Belgian nuclear engineering firm Belgo-nucleaire would act as architectural engineer on the project and dish out another $967 million in nonnuclear contracts, presumably to Belgian companies. This is tempting to a country with the highest unemployment rate in Western Europe.

''No militarily sensitive information would change hands,'' says a senior executive at Belgonucleaire.

But the Reagan administration fears an atomic reactor could be the first fateful step toward Libya manufacturing nuclear weapons. US officials have been asking whether Qaddafi would learn from his contacts with Belgonucleaire and other companies how to reprocess used nuclear fuel. Reprocessing yields plutonium, the central element needed in building atom bombs.

Officials at the Vienna-based International Atomic Energy Agency point out that under a safeguards agreement signed with the organization in 1980 Libya is obliged to open its nuclear facilities to international inspection to ensure they are not being used for military purposes.

The Libyan news agency JANA reported last week that ''serious talks'' were under way with the USSR for construction of the nuclear power plant. It is assumed this referred to work already contracted for. But the prospect of Libya turning to Moscow for all its nuclear needs would raise some eyebrows in Washington.

An official of Belgium's nuclear industry said: ''We think we should be in Libya, if for no other reason than to check up on what the Russians are doing.''