GROZNY, Russia (AP) _ Six Russian policemen held hostage in the breakaway republic of Chechnya have been released in exchange for three detained members of an Islamic sect, ITAR-Tass news agency reported Wednesday.

The officers were kidnapped in December during a Chechen guerrilla raid in the neighboring region of Dagestan, according to ITAR-Tass.

The six were exchanged Tuesday for three detained Wahhabis, Islamic militants operating in Chechnya, the news agency reported.

Scores of ethnic Russians and foreigners have been kidnapped in or near Chechnya since the end of the 1996 independence war with Russia.

Dagestan, in particular, has been destabilized by frequent clashes and kidnappings, many of them linked to Chechen gunmen or militant Wahhabis.

In the latest violent incident on Wednesday, unidentified attackers fired grenades at the offices of the mayor of Dagestan's capital, Makhachkala.

The mayor, Said Amirov, was not hurt in what was the seventh attempt on his life. A previous attack left him partly paralyzed.

Police injured one of the assailants, but failed to capture them. No group has taken responsibility for the attack.

Officials deployed armored vehicles in Makhachkala's central square following the incident.

Chechnya has had de facto independence since Russian troops pulled out, but no country recognizes the claim.

Russian media have warned that recent violence in and around Chechnya could trigger large-scale hostilities, but Russian Interior Minister Sergei Stepashin insisted the government is in control of the northern Caucasus.

Meanwhile, Chechen President Aslan Maskhadov used the state of emergency he introduced earlier this month to ban private television channels on Wednesday, many of which supported his political opponents.