UNITED NATIONS (AP) _ The U.N. Secretariat has discontinued informal contacts with nationalist leaders of the three Soviet Baltic republics but U.N. officials said Thursday no pressure came from the Soviet Union.

Francois Giuliani, spokesman for the secretary-general, said in a briefing Thursday the contacts were strictly ''informational'' between the nationalist groups of Estonia, Latvia and Lithuania and Assistant Secretary-General James O.C. Jonah.

He said they went on for several months and were initiated by the groups. Secretary-General Javier Perez de Cuellar had not been a party and had not been informed until recently, he added.

In answer to a question, Giuliani said Soviet officials had not sought to intervene or block contacts.

Soviet President Mikhail S. Gorbachev currently is visiting Lithuania trying to diffuse separatist sentiment there.

U.N. officials said Soviet Deputy Foreign Minister Vladimir Petrovsky contacted Jonah's office about the discussions, but lodged no protest with the secretary-general.

The Soviet Mission's press officer was not available for comment, although a message was left with his office by telephone.

The U.N. Charter bars the world organization from interfering in members' domestic affairs.

Estonia, Latvia and Lithuania, once independent, were forcibly annexed by the Soviet Union in 1940 and never have acquiesced to Soviet domination. Baltic nationalism has been fanned by democratic upheavals in Eastern Europe and by policies of openness and reform under Gorbachev.

The Soviet Constitution provides the right of secession to its republics but Gorbachev opposes any breakup of the Soviet Union.

Baltic groups in New York expressed regret over the U.N. decision.

''We are sorry to see that these informal, informational contacts have been discontinued because of Soviet sensitivity,'' said Ginte Damusis of the Lithuanian Information Center.

''It is scandalous to see the United Nations officials running scared of Moscow.''