GENEVA (AP) _ Superpower arms talks resumed Tuesday after a seven-week break, and the chief U.S. negotiator played down hopes for a breakthrough on a treaty slashing long-range nuclear arsenals.

Disagreement at the negotiations, which began in March 1985, centers on U.S. plans for a space-based missile defense and Soviet insistence on limiting sea-launched cruise missiles. Many anti-cheating provisions also remain open in a draft agreement of more than 300 pages.

U.S. and Soviet teams held a two-hour plenary meeting at the U.S. mission to launch the 10th round of talks, for which no end has been set. The talks had recessed for the Moscow summit.

A source close to the talks who demanded anonymity called the initial two- hour session ''productive'' but declined to say if new proposals were made. ''Some interesting things were said,'' the source said, refusing to elaborate.

Before the opening session, chief U.S. negotiator Max Kampelman reaffirmed Washington's stand that the draft pact halving each side's strategic stockpile ''should not be held hostage'' to limits sought by Moscow on President Reagan's missile defense program, the Strategic Defense Initiative.

Asked whether a possible fifth superpower summit or the end of Reagan's presidency in January could be target dates for agreement, Kampelman said talks will ''continue no matter who is president and no matter who the leader of the Soviet Union is.''

''I don't think important issues should be governed by the calendar,'' he told reporters before the opening session. ''If it takes longer fine, if it takes shorter fine.''

Kampelman said on Monday that an agreement was still possible under the Reagan administration. He and his top two aides were to brief NATO allies on Wednesday in Brussels on the start of the talks. The negotiations are to continue Wednesday or Thursday, the U.S. side said.

At the May 29-June 2 summit in Moscow, Reagan and General Secretary Mikhail S. Gorbachev ordered negotiators to continue ''energetically and purposefully'' after the two leaders made only limited progress on arms control.

Kampelman told reporters Monday: ''Don't look for drama. We're at a stage now where hard work is required rather than drama.''

Soviet chief delegate Alexei Obukhov said on Sunday the talks must not lose momentum.

Both said the talks have come ''a long way'' but Kampelman noted much still remains to be resolved.

Washington and Moscow agree the proposed Strategic Arms Reduction Treaty should cut each side's stocks over seven years to an overall limit of 6,000 nuclear warheads and 1,600 launchers, such as heavy bombers and land-based intercontinental missiles.

Some details of monitoring compliance are also set.

But Obukhov said progress was imperative on a renewed U.S.-Soviet commitment to the 1972 Anti-Ballistic Missile Treaty, which Moscow says is threatened by Strategic Defense Initiative, also known as SDI.

Reagan says SDI does not violate the ABM pact, and Kampelman reaffirmed the U.S. side will not allow Moscow ''to use these negotiations to have us end our Strategic Defense Initiative.''

The Geneva negotiators have yet to agree on a joint text pledging each side to observe the ABM treaty for a certain period.

Obukhov also pressed for limits on long-range, sea-launched cruise missiles and verification measures including inspections of warships.

Washington has balked, saying the weapons cannot be reliably detected. U.S. START negotiator Steven Hanmer called Soviet verification proposals ''totally impractical'' and said current prospects for resolving the issue are dim.