By Douglas Busvine and Alissa de Carbonnel

MOSCOW (Reuters) - A Boeing 737 airliner crashed on Sunday in the Russian city of Kazan, killing all 50 people on board and spotlighting the poor safety record of regional airlines that ply internal routes across the world's largest nation.

The son of the president of the oil-rich province of Tatarstan and the regional head of the FSB intelligence service were named among those killed when the plane exploded in a ball of fire on hitting the runway.

Pictures showed charred wreckage scattered over a wide area, apparently taken after firefighters had extinguished the fire. Russian television broadcast a blurred video showing a bright flash of light. It also published a photo of the plane's gaping fuselage with firefighters in the foreground.

The Tatarstan airlines flight from Moscow had been trying to abort its landing in order to make a second approach when it crashed, killing all 44 passengers and six crew on board, emergency officials said.

Flight U363 took off from Moscow's Domodedovo airport at 6:25 pm (1425 GMT) and crashed just over an hour later, emergency officials said. The leased plane was 23 years old.

According to local reports, the Boeing lost altitude quickly and its fuel tank exploded on impact.

There were high winds and above-zero temperatures over the airport in central Russia. Flights to and from the airport were halted until midday on Monday.

Kazan, which is 800 km (500 miles) east of Moscow, is the capital of the largely-Muslim, oil-rich region of Tatarstan. A new runway was built at the airport ahead of the World Student Games, held in the city earlier this year.

Russia will host the Winter Olympics in the southern city of Sochi early next year.

The son of Tatarstan President Rustam Minnikhanov, Irek, was among those killed in the crash, as was the head of the regional Federal Security Service (FSB) Alexander Antonov, according to a passenger list whose authenticity was confirmed by the regional government.

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Russia and the former Soviet republics combined have one of the world's worst air-traffic safety records, with a total accident rate almost three times the world average in 2011, according to the International Air Transport Association.

Prime Minister Dmitry Medvedev called the disaster "a frightening tragedy", offering his condolences to the relatives of the victims in a Tweet on Sunday.

State television showed images of a woman scanning a list of passenger names posted outside the airport and crumbling into tears as she apparently recognized one.

Boeing officials had no immediate comment on the circumstances of the crash, but issued a statement.

"Boeing's thoughts are with those affected by the crash of the Tartarstan aircompany flight. Boeing is prepared to provide technical assistance to the investigating authority as it investigates the accident."

SPRAWLING COUNTRY

Russia spans nine time zones, from the Baltic Sea to the Pacific across large areas of largely uninhabited land, making efficient air and train links especially important to the country's economy.

In Soviet times, flag carrier Aeroflot had a virtual monopoly of the airline industry, but after the collapse of the Soviet Union, a multitude of small private companies emerged.

A spokesman for state aviation oversight agency Rosaviatsia said authorities would search for the flight recorders.

"The plane touched the ground and burst into flame," Sergei Izvolsky said. "The cause of the crash as of now is unknown."

The plane had been forced to make an emergency landing a year earlier on November 26 due to problems with "cabin depressurization" shortly after take off, a law enforcement source told Interfax news agency. No one was hurt.

IATA said last year that global airline safety had improved but that accident rates had risen in Russia and the ex-Soviet Commonwealth of Independent States.

In April 2012, at least 31 people were killed when a Russian passenger plane crashed after take-off in Siberia.

In September 2011, a Yak-42 passenger jet carrying members of a major league ice hockey team came down shortly after takeoff and burst into flames near the Russian city of Yaroslavl, killing 44 people.

The Boeing 737 is the world's most popular passenger jet in commercial use today. There have been 170 crashes involving this model of aircraft since it came into use.

In the Russian city of Perm in 2008, a Boeing 737 exploded a kilometer above the ground, killing 88 people.

(Reporting by Douglas Busvine and Alissa de Carbonnel; Additional reporting by Gleb Stolyarov and Tim Hepher in Dubai; Editing by Ralph Boulton)