A Soviet-made passenger plane was shot down by an Afghan guerrilla surface-to-air missile over northern Afghanistan, killing 23 passengers and six crew members, amid new rebel threats to step up attacks on Soviet forces.

There was no independent confirmation of the report Sunday on state-run Afghan radio, which said the An-26 passenger plane was hit while on a domestic flight from Maimana, provincial capital of Bamiyan, to Mazar-i-Sharif, provincial capital of Balkh. Both provinces border the Soviet Union.The report said "extremists" firing a surface-to-air missile downed the aircraft about 3:10 p.m. Sunday. Among the 23 passengers killed were eight women and two children, the broadcast said. Six crew members also were killed, it said.

Soviet-installed Afghan leader Najibullah extended his condolences to relatives of the victims, the report said.

Gulbuddin Hekmatyar, chairman of the alliance of seven major Western-backed guerrilla groups, told a news conference at the alliance headquarters in the frontier town of Peshawar Sunday that attacks on Soviet forces would escalate in coming days.

The warning follows the announcement in Geneva last week by U.N. mediator Diego Cordovez that a Soviet withdrawal accord between Pakistan and the Kabul government endorsed by Moscow and Washington would be signed this week.

The agreement, six years in the making, calls for a pullout of the Soviet Union's 115,000 occupation troops within nine months. Soviet leader Mikhail Gorbachev has announced the withdrawal will begin by May 15.

The talks excluded the Afghan resistance, which demanded direct negotiations with Moscow.

"There will be a major military offensive now, bigger than any time before, to show the world that without the consent of the Mujahideen no agreement can be consummated," the hard-line fundamentalist leader said.

"We don't believe they are pulling out their troops because of the discussions (in Geneva). They are pulling out because they were unable to crush the Afghan people," Hekmatyar said.

The guerrillas have rejected overtures from Najibullah for a government of "national reconciliation" and remain determined to topple his regime.

Sunday's report of the U.S.-backed Moslem rebels shooting down a passenger aircraft was the first since June 1987, when two Antonov-26 short-range airlift transports were shot down during a two-week period, killing 98 people, according to Western diplomats and Afghan officials.

The two aircraft last year were reported downed by U.S.-supplied shoulder-fired Stinger missiles but Sunday's broadcast said only that a surface-to-air rocket was used.