BRUSSELS, Belgium (AP) _ NATO defense ministers today began three days of talks that officials said would focus on upgrading the alliance's conventional forces and the sensitive issue of arms cooperation.

Twelve European defense ministers of the North Atlantic Treaty Organization began meeting at 10 a.m. (4 a.m. EST). They will be joined Tuesday and Wednesday by U.S. Defense Secretary Caspar Weinberger and Harvie Andre, his Canadian counterpart.

Weinberger is due to arrive in Brussels tonight.

Before the start of the meetings at NATO headquarters, officials said the ministers would review cooperation among allied nations in weapons research and production. The issue is a sensitive one at a time when many NATO nations worry about keeping their own arms industries in business.

Under legislation introduced by U.S. Sen. Sam Nunn of Georgia, the United States will make $250 million available to promote cooperation among the allies in research and development programs.

The issue of greater arms cooperation within NATO is also due to be discussed by the NATO foreign ministers who meet here next week.

NATO officials say the allies can save money and cut wasteful duplication by pooling resources in joint arms production schemes.

Most NATO allies have in recent years failed to reach the NATO goal of increasing defense spending in real terms by at least 3 percent annually.

This has worried U.S. General Bernard W. Rogers, NATO's top European commander, who warned recently that unless the allies improve their conventional defenses, he would have to quickly resort to nuclear arms in time of war.

When all the defense chiefs meet starting on Tuesday, they will receive a report by Dutch General Cornelius de Jager, the chairman of NATO's Military Committee, on the state of NATO forces compared with those of the Warsaw Pact.

The report from the Military Committee, a panel of senior allied commanders, is annually updated and always forms part of the ministers' year- end meetings.

Also on Tuesday, the ministers will be briefed by NATO strategists on the alliance's long-term outlook of the Soviet military posture.

In recent months, the NATO staff has worked out a so-called Conceptual Military Framework which takes stock of current force levels on the NATO side, identifies weaknesses and suggests areas where improvements are most needed.

The 12 European defense ministers who met here today represented Britain, West Germany, Italy, Belgium, the Netherlands, Luxembourg, Norway, Portugal, Turkey, Greece, Denmark and Spain.

France and Iceland did not take part. Iceland has no armed forces and France is not part of NATO's integrated military command. The two countries participate only in the meetings of the NATO foreign ministers.