MOSCOW, MARCH 3 -- Ethnic upheaval spread from riot-torn Sumgait to another city in the Soviet republic of Azerbaijan, a Soviet official said here today. For the fourth time in less than two weeks, special militia were called in to quell the disturbances.

Residents demonstrated in Kirovabad -- Azerbaijan's second-largest city -- amid rumors about ethnic protests in a disputed area 30 miles away, the official said.

The demonstrations, now reportedly under control, mark the fourth outbreak in the Caucasus region in the past two weeks. In addition to Sumgait, a port on the Caspian Sea, there have been protests in Yerevan, the capital of the neighboring republic of Armenia, and the district of Nagorno-Karabakh, a mostly Armenian enclave within Azerbaijan.

The situation in each of those areas is now under control, Foreign Ministry spokesman Gennadi Gerasimov told reporters today. He said a nonvoting member of the ruling Politburo has been sent to Sumgait, the scene of bloody riots on Sunday, and a commission has been set up to investigate the violence.

In Sumgait, near the Azerbaijani capital, Baku, a curfew policed by Soviet troops in Army personnel carriers was expected to be lifted today, a local official told reporters.

In Nagorno-Karabakh, an official ultimatum to suspend three weeks of street protests has been heeded, dissident sources in Yerevan said. Ethnic Armenians there have demanded that Nagorno-Karabakh be joined to Armenia.

In Yerevan, where hundreds of thousands demonstrated in support of Armenians in Nagorno-Karabakh, Armenian sources reached by telephone said work has returned to a normal pace. Armenian nationalists, who started the protests, are awaiting a decision by Communist Party leader Mikhail Gorbachev, who said last week that he was reviewing the causes of the disputes.

At a news briefing today, Gerasimov gave some details about the disturbances, in which up to 17 were reportedly killed. The riots and deaths make it the biggest ethnic clash in decades in the Soviet Union.

"Unbalanced people got involved in them under the influence of provocative rumors and incendiary talk about what had taken place in Nagorno-Karabakh," Gerasimov said. "Criminal elements perpetrated bandit actions {amid} rampant emotions," he added, "and there was loss of life."