BRUSSELS, FEB. 17 -- British Prime Minister Margaret Thatcher, citing a Soviet threat to the unity of the western alliance, urged the North Atlantic Treaty Organization today to stiffen its defenses by updating its nuclear weapons.

Thatcher, on her first official visit to NATO headquarters, came out strongly in favor of modernizing short-range nuclear forces, a key issue that could split a summit of alliance leaders here early next month.

"You don't deter with obsolete weapons," she told reporters after addressing a closed meeting of ambassadors to NATO.

"You don't, as an alliance, put your troops in the field with weapons less good than those they face. Modernization is a part of deterrence, it is a part of defense. It is totally and utterly absurd to separate out the modernization of nuclear weaponry from the modernization of anything else."

West Germany, which borders Warsaw Pact nations, has expressed misgivings about pressing ahead with a 1983 NATO program to modernize short-range nuclear missiles.

Alone among the 16-member alliance, the Bonn government wants early talks with the Soviet Union on reducing such weapons, which are mainly deployed by NATO on West German soil.

Short-range missiles with a range of up to 300 miles are not covered by the Intermediate-range Nuclear Forces treaty signed by the United States and the Soviet Union in December.