Four armed hijackers on Thursday took 40 bus passengers including several children hostage near the town of Mineralniye Vody in southern Russia, demanding $15 million and two helicopters in the fifth such attack in the region since December.


Itar-Tass quoted police as saying that four men, armed with two pistols and a large number of hand grenades, boarded an afternoon bus in the town of Pyatigorsk. When the bus stopped at a terminal near the airport of Mineralniye Vody, the men suddenly pulled black masks over their heads and took the passengers hostage, NTV evening news reported.


They released two women to pass on their demands, and later received a field telephone to converse with police. Seven more hostages were released later, according to a government official quoted by Reuters.


A special anti-terrorist squad, which has foiled several hijackings in the past without harming the hostages, rushed to the airport, Itar-Tass reported.


State television said the incident was the seventh kidnapping since 1988 in Mineralniye Vody, a Caucasian resort town bordering the breakaway region of Chechnya and a patchwork of other unruly republics and regions. The town was declared an emergency zone in President Boris Yeltsin's recent decree on fighting crime.


The attack appeared a carbon copy of two earlier bus hijacks in Mineralniye Vody, on May 26 and June 28. In May, four gunmen traded a busload of hostages for a helicopter and a ransom said to be between $4 million and $10 million. In June, police paid $2.8 million in ransom to three hijackers in a similar attack. Both attempts were foiled when the kidnappers fled to their native Chechnya. The trial against three of the May kidnappers started Wednesday.


In December, four gunmen kidnapped a dozen schoolchildren and four adults in Rostov-on-Don and shuttled them in a helicopter across Russia for five days, twice passing through Mineralniye Vody. They received $10 million in ransom but were arrested after a gun battle with police.


In February, a man took children and adults hostage in southern Ukraine but was quickly disarmed and captured by special anti-terrorist forces, just as he attempted to board a plane with five hostages in Rostov-on-Don.


Three Iranians who forced a Russian plane to divert to Norway last September earlier this month appealed a Norwegian court decision to have them extradited to Russia.


Hostages were freed unhurt in all but one incident. In 1988, a famous family of musicians -- known as the Seven Semyonovs -- hijacked a plane on a flight from Irkutsk to Leningrad. Two hostages and several hijackers were killed when troops stormed the plane at a military airport near Leningrad.