Scores of heavily armed gunmen believed to be Chechen rebels attacked a southern city Wednesday and seized up to 300 hostages after furious gun battles with security forces that left at least 15 dead.


Russian military authorities placed all their forces in the North Caucasus on full battle alert as officials said the gunmen, who demanded a halt to Russian military operations in the breakaway republic of Chechnya, were trying to retreat from Budyonnovsk with their hostages in captured buses.


Budyonnovsk is located in the Stavropol region, some 200 kilometers north of the rebel republic.


The attackers threatened to kill the hostages "unless the Russian military immediately stops hostilities in Chechnya,'' according to Itar-Tass.


There were street battles as the gunmen, wearing camouflage army-type uniforms, attacked government installations and took hostages, news reports said. Houses and cars were on fire in the city of some 100,000 people.


City officials said there were at least 15 dead and 21 wounded on both sides. The Russian government later reported 20 police officers killed, and Interfax said 17 gunmen and at least 20 civilians had died.


Itar-Tass said First Deputy Prime Minister Oleg Soskovets announced the state of alert at an emergency meeting of law enforcement organs. Soskovets also said all airports in the region would be shut from 0.15 a.m. Thursday, the agency reported. He said the attackers were demanding the withdrawal of Russian troops from Chechnya.


The announcement will inevitably give rise to speculation that the Kremlin could soon clamp a state of emergency over the entire region.


Russian television screens pictured smoking ruins and several dead people on the streets of Budyonnovsk, with helicopters flying overhead and armored vehicles belonging to security forces stationed on road junctions. The assault on Budyonnovsk was one of the worst terrorist attacks in recent Russian history. Chechen rebel commanders had threatened to carry the war to Russia if Moscow did not stop the fighting.


"We've never had anything similar,'' Interior Ministry spokesman Yevgeny Ryabtsev said.


Gunmen firing from a hill overlooking the town hit civilians in the streets, and dead and wounded people were lying on the central square and around apartment blocks, said Sergei Yushenkov, a legislator who heads the Russian parliament's security committee.


"They have staged a real hunt for civilians. They fired at them, tried to run them over with their trucks,'' Ryabtsev said of the attackers.


A duty officer at Federal Security Service (FSB) headquarters in Moscow, Alexander Korcheba, said the gunmen seized at least 200 hostages, most of them civilians. But other security sources said there could be up to 300 hostages.


The attackers numbered about 100, divided into disciplined groups of 10 to 15 men equipped with weapons and radios, security officials said.


President Boris Yeltsin, the FSB and the military also blamed the attack on rebels from war-torn Chechnya. "The president is deeply concerned by the terrorist attack of Chechen militants in the Stavropol region, which resulted in casualties, hostages, and disruption of the constitutional order,'' said Yeltsin's spokesman Sergei Medvedev.


Chechen separatist President Dzhokhar Dudayev has repeatedly warned of possible revenge attacks.


Reports of the early afternoon attack were often confusing and sketchy.


The gunmen, who arrived in two large trucks reportedly accompanied by a stolen police car and were armed with assault rifles, grenade launchers and machine guns, attacked city hall, police headquarters, a bank and a local communications center, police said.


They also apparently attacked the town's military helicopter airfield.


The assailants placed a large-caliber machine gun on the roof of one building and that of a local hospital, threatening to kill some 60 patients and doctors inside, the Russian government's information department said.


At least 10 police officers were killed, dozens of civilians killed and wounded, four military personnel injured in a helicopter downed by the gunmen and dozens of the assailants were killed, Itar-Tass reported.


Itar-Tass reported a particularly violent firefight around the police headquarters, where officers barricaded inside were "fiercely resisting'' attempts to storm the building.


Local officials also reported exchanges of fire around a major chemical plant, one of Russia's largest producers of plastics. Itar-Tass said the attackers originally planned to blow up the plastics plant and "cause an ecological catastrophe'' in the region.


Police reinforcements were rushed to the city. Large forces of Interior Ministry troops also began arriving. Interior Minister Viktor Yerin and FSB chief Sergei Stepashin left Moscow for the region.


By evening, several buses and cars with hostages and kidnappers inside reportedly remained in the city, where the attackers also continued to hold the hospital building.


Other gunmen were retreating in captured buses toward Mineralnye Vody, about 100 kilometers southwest of Budyonnovsk.