Negotiators Working on Treaty Snags : Arms control: Most decisions have been made but some missile issues remain unresolved.

U.S. and Soviet arms-control negotiators caucused separately Monday in preparation for additional talks today aimed at reaching a long-range nuclear weapons agreement that President Bush and Soviet President Mikhail S. Gorbachev can sign at their Washington summit.

The Soviet delegation, led by Deputy Prime Minister Viktor P. Karpov, sought additional instructions from Moscow on Monday after meeting for 6 1/2 hours Sunday with a U.S. delegation led by Undersecretary of State Reginald Bartholomew.

Bush and Gorbachev are expected to reach agreement on the broad outlines of a strategic arms reduction treaty (START) during the summit, which is to begin Thursday. But it will probably take the rest of the year to fill in all the details.

No information was made public about the Memorial Day weekend talks beyond terse announcements that meetings had been held. Final decisions are not expected until after Gorbachev arrives Wednesday.

Most of the basic decisions have already been reached on the pact, which is intended to cut the superpower nuclear arsenals by about one-third. The last major disputes, over sea-launched and air-launched cruise missiles, were settled earlier this month when Secretary of State James A. Baker III visited Moscow for talks with Gorbachev and Foreign Minister Eduard A. Shevardnadze.

However, a few troublesome issues were left unresolved, including a controversy over Soviet plans to modernize the giant SS-18 ballistic missile. Moscow has already agreed to cut its arsenal of SS-18s in half, but it wants to retain the option of updating the remaining missiles. Washington opposes any modernization.

The SS-18 is the only missile in its class. The United States has no weapons of similar throw-weight.

In addition, disputes remain over limits on mobile missiles, verification of restrictions on road-mobile missiles, classification of the Soviet Backfire bomber and the wording of a statement banning efforts to circumvent the treaty.

U.S. officials also said the arms-control negotiators are trying to bridge the gap between Washington and Moscow over non-nuclear arms in Europe.

These talks on conventional arms in Vienna involve all 23 members of the North Atlantic Treaty Organization and the Warsaw Pact. But the United States and the Soviet Union are clearly the most influential of the participants, and an agreement between the superpowers probably would produce an agreement between the two blocs.

Meanwhile, U.S. and Soviet negotiators completed work on an agreement to expand commercial airline service between the two countries. Bush and Gorbachev are expected to sign the pact later this week.

The agreement would allow six additional U.S. airlines to join Pan American World Airways in providing service between the two nations. An additional Soviet airline would also be permitted.