SERNOVODSK, Russia -- About 250 Russian soldiers were killed early Friday when their unit was surrounded and overrun by Chechen insurgents in heavy fighting, a top official of a neighboring republic said.

Ali Dudarov, deputy interior minister of the republic of Ingushetia, said Russian officers told him that the unit was attacked near the Chechen town of Urus-Martan, 12 miles southwest of the capital Grozny.He said some 200 soldiers were killed in the attack and 50 more who had been taken prisoner were put to death when their throats were slit.

In a separate incident, about 40 Chechen refugees were killed when Russian troops fired on a refugee convoy near the Chechen capital of Grozny, according to survivors.

Russian officials closed the border crossing from Chechnya to Ingushetia today, saying power supply problems made it difficult to check refugees' documents. The border point is the main exit out of Chechnya.

But Dudarov said officers at the border told him the real reason for the border closing was the destruction of the Russian unit. If true, the incident would be the biggest loss suffered so far by the Russian military in the current campaign and a major setback for the Russian forces.

The Russian government declined to comment on the report.

In the other incident, Russian troops opened fire without warning early today on a refugee convoy near the village of Goity just outside Grozny, raking a bus and several cars with bullets, survivors said.

"They shot at us point blank and they burned the bus. Forty people were killed and seven were wounded," said Tais Aidalarova, a survivor interviewed at a hospital in Sleptsovsk in Ingushetia.

Several survivors gave similar accounts of the attack, including saying that the Russians gave medical help to the wounded.

The reported Russian losses at Urus-Martan came as Russian forces claimed they had achieved a major victory by seizing the key town of Argun, three miles east of Grozny. Argun was a key target in the Russian drive to surround Grozny and it has been the target of Russian air and ground attacks for more than a week.

But Chechen commanders said their forces were still fighting in the area and the ITAR-Tass news agency, citing Russian military officials, reported there was heavy fighting in Argun.

A senior Russian commander said three federal soldiers were killed and 34 wounded in the fight for Argun, while claiming 100 Chechen insurgents were killed and 400 more fled. Russian claims that they suffered almost no casualties while inflicting heavy Chechen losses did not appear credible.

"Those who couldn't save themselves by running were destroyed under the advance of the Russian troops," said Gen. Valery Manilov, according to the Interfax news agency.

Manilov said the Russian troops started a sweep of the city at daylight today. He said the military would direct its next attacks on Chechnya's mountainous south, where the rebels have their main strongholds.

Russian commanders admitted earlier this week that they were running into growing resistance and it could take months to defeat the Chechen forces.

The Russian forces are not prepared to risk a major ground attack on Grozny. Such an attack would almost certainly result in heavy casualties and repeat images of the bloody 1994-96 Chechen war that the military has been eager to avoid.

The military has relied on airstrikes and artillery shelling, coupled with small ground battles, to seize ground. Russian forces have surrounded Grozny from the north, west and east.

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The Russians again shelled Grozny with air and artillery strikes during the night, hitting several television transmitters, Interfax said. Russian helicopter gunships and jets also hammered Urus-Martan.

The attacks continued to send refugees fleeing west to Ingushetia, where cold-related illnesses were becoming an increasing problem as temperatures hovered below freezing. So far, some 233,000 people have fled Chechnya to escape the fighting.

Russian forces entered Chechnya in September following incursions by the militants into neighboring Dagestan and apartment bombings in Russian cities that left 300 people dead. Russia has blamed the militants for the bombings.