Two Russian passenger planes carrying more than 80 people have crashed almost simultaneously after both planes took off from the same Moscow airport. Interfax news agency quoted a government source on Wednesday as saying one plane carrying more than 40 passengers and crew sent a hijack alarm before crashing near the southern Russian city of Rostov-on-Don. The planes disappeared within minutes of each other and Russian news agencies said President Vladimir Putin had ordered the FSB security service to investigate the incidents - something it would do only under suspicious circumstances. Witnesses on the ground saw an explosion on board the second plane just before it crashed near Tula 150km south of Moscow. "Around 11 pm, give or take five minutes, there was this strange noise in the sky, then this torn-up book fell onto our garage," a local man told NTV television, holding up the book with its tattered pages. **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 can not exclude a terrorist act." The unnamed source told Interfax FSB and police experts were working in Domodedovo airport - where 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 fighters have been battling Russian forces for a decade. Chechen resistance leader, Aslan Mashkadov, however, has denied any connection to the crashes. "Maskhadov is not linked to this in any way," said Ahmad Zakayev, his spokesman, who has won refugee status in Britain, and now lives in London. Maskhadov was elected Chechen president in 1997, after the republic won de facto independence from Russia, but has since been branded a "terrorist" by Russian President Vladimir 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. **Rescue efforts*** An Emergency Ministry spokeswoman said there were no survivors as it fell from 10,000 metres. Up to 500 rescue workers and police were at the scene, officials said. Just three minutes after the Tu-134 crashed, air traffic controllers lost contact with the other passenger plane, a Tu-154 bound for the Black Sea resort of Sochi. Its wreckage was not found until Wednesday morning, the search being disrupted by heavy rain. Interfax quoted major Russian air carrier Sibir, the owner of the plane, as saying 46 people including eight crew were on board the plane, in operation since 1982. **PHOTO CAPTION*** Russian Emergency Ministry soldiers walk near debris of a Russian Tu-134 airliner carrying 43 passengers, which crashed near the village of Buchalki in the Tula region, about 200 kilometers (125 miles) south of Moscow early Wednesday morning, Aug. 25, 2004. (AP)