MOSCOW, JUNE 6 -- The chairman of the Soviet legislature said today that army reinforcements had been sent to the Central Asian republic of Kirghizia from nearby Turkmenia to quell three days of ethnic riots, which he said had taken about 40 lives.

The turmoil in Kirghizia poses a new crisis for President Mikhail Gorbachev, who flew back to Moscow Tuesday night following his visit to the United States and Canada. He will host a Warsaw Pact summit meeting here Thursday and must also give urgent attention to the secessionist challenge from the Baltic republics, moves by the Russian republic to assert its sovereignty and mounting opposition to the government's economic plans.

Legislative chairman Anatoly Lukyanov said fighting was still raging between rival gangs of Kirghiz and Uzbeks in and around the Moslem holy city of Osh in southern Kirghizia. Soviet authorities declared a state of emergency in the region Tuesday, and police in the republic's capital, Frunze, fired shots into the air today to disperse crowds throwing stones at local Communist Party leaders.

The decision to use army units in Kirghizia indicates the gravity of the Soviet Union's latest ethnic flareup and the inability of internal security forces to cope with the violence. The Kremlin has been reluctant to use the army for policing duties since the brutal crushing by the military of nationalist demonstrations in Tbilisi, the capital of Soviet Georgia, in March 1989.

Meanwhile, the presidents of the Lithuania, Latvia and Estonia met today to discuss the results of the U.S.-Soviet summit, and officials said the three agreed to send a letter to Gorbachev urging him to open talks on independence for their republics and rejecting his demand that the issue be put to referendums.

On other matters, the Soviet news agency Interfax reported that the Communist Party's policy-making Central Committee would meet this weekend, purportedly to review preparations for next month's key party congress. But the meeting also could provide an opportunity for Gorbachev's critics in the leadership to air their grievances.

Interfax said the committee could discuss a new version of the government's program for a transition to a market economy, which has come under attack from radicals and conservatives alike. The plan, which sparked panic buying last month as soon as it was unveiled, is widely regarded as unworkable because of the opposition of the legislatures of Russia, the Ukraine and Byelorussia, the Soviet Union's Slavic heartland.

At a news conference here Tuesday, a member of Gorbachev's presidential advisory council, Veniamin Yarin, suggested that negotiations should be held between leaders of the republics and Kremlin officials before the Soviet legislature is asked to vote on the plan. The legislature, which is now scheduled to adjourn June 14, has already postponed debate on the plan following the outpouring of public opposition.

The disturbances in Frunze today started after thousands of Kirghizian students began gathering in several parts of the city, including the university and the central square, to demand restoration of telephone links with Osh and the dispatching of a volunteer Kirghizian force to the city.

The official Tass news agency said Kirghizia's Communist Party chief, Medet Sherimkulov, was met by a hail of stones when he tried to address a mass rally in the city and that "to avoid aggravating the situation, police were forced to fire warning shots in the air." Residents of Frunze reported that several Uzbek tradesmen were beaten up at the local market, forcing its closure, and that many Uzbeks in the city were being evacuated.

The immediate cause for the latest surge in Soviet ethnic violence was a dispute over land allocation between Kirghiz and Uzbeks in Osh, a center of pilgrimage for Moslems from all over Central Asia. But it was quickly transformed into a political dispute as members of the large Uzbek minority in the Osh district began pressing for creation of their own autonomous region.

In a television address Tuesday night, Kirghizian President Absamat Masaliyev accused Uzbek opposition groups of making "ultimatum-like demands." The Osh district borders the Fergana Valley region of western Uzbekistan, where more than 100 people were killed in ethnic disturbances last year involving Uzbeks and Meskhetian Turks.

Interfax reported that rival gangs of Uzbeks and Kirghiz had attempted to seize police stations and internal-security force outposts in five districts around Osh and said police opened fire to repel the attacks. At least four policemen and three soldiers were reported injured in the assaults.

Lukyanov told legislators that the total casualty toll from three days of rioting now stood at about 40 dead and more than 200 injured. More than 370 people have been killed in ethnic violence in the Soviet Union this year.

According to Tass, Soviet troops and armored personnel carriers were patrolling Osh and setting up checkpoints today as military helicopters hovered overhead.

Officials also are concerned that the ethnic clashes in Kirghizia could spill over the border into Uzbekistan. About 1 million Kirghiz live in Uzbekistan, the most populous of the Soviet Union's Central Asian republics.