Russia's top investigator on Wednesday said police were looking at human error, mechanical mishap or a possible Chechen separatist involvement.

But as yet, prosecutor General Vladimir Ustinov has told President Vladimir Putin he has no clear view of what happened to the planes, which took off from Moscow's Domodedovo airport late on Tuesday around an hour apart for two different destinations.
   
A Tu-134 flying to Volgograd went down near the town of Tula south of Moscow. Seconds later, and 800km away, a Tu-154 bound for the Black Sea resort of Sochi crashed near the southern town of Rostov-on-Don.
   
Possible explosion

The owner of the Tu-154, Sibir Airlines, said the pilots had triggered a hijack alert just before their plane crashed. It was carrying 46 passengers and crew.    

President Putin has ordered the
security service to investigate

"The message was generated right before all contact was lost with the plane and it disappeared from radar screens," Russia's number two airline said in a statement. The wide distribution of wreckage suggested the plane exploded in mid-air, it said.
   
Volga-Aviaexpress, a small regional carrier which owned the Tu-134, said the crew did not report any problems on board before the plane crashed with 43 passengers and crew. The Emergencies Ministry later said 44 people were aboard.

Security tightened
 
Local news agencies reported that security had been tightened at all the country's airports after the incidents.

"The fact that both planes took off from one airport and disappeared from radars around the same time can show it was a planned action," Interfax quoted an aviation source as saying.

"In such a situation one cannot exclude a terrorist act."

The unnamed source told Interfax that FSB and police experts were working in Domodedovo airport - from which both planes took off - to identify whether all passengers had undergone proper security checks.

Chechen denial

The incidents came just days before a presidential election in Chechnya where separatists have been battling Russian forces for a decade. 

The Tupolev Tu-154 jet crashed
near Rostov-on-Don

Chechen separatist leader, Aslan Maskhadov, however, has denied any connection to the crashes.

"Maskhadov is not linked to this in any way," said his spokesman Ahmad Zakayev.

Maskhadov was elected Chechen president in 1997 after the republic won de facto independence from Russia, but has since been branded a "terrorist" by Putin.

The plane which crashed near Tula was a Tu-134 airliner with 35 passengers and seven to nine crew on board, Russian news agencies quoted Volga-Aviaexpress carrier, which owned the plane, as saying.

Airport officials said it had nearly reached cruising height. The company said the plane was in good shape and its passengers had undergone all necessary security checks.