Malaysia Airlines  plane crash Ukraine
People stand amongst the wreckages of the Malaysian Airlines plane. Getty Images

Armed pro-Russian separatists at the crash site of the Malaysian Airlines plane MH17 are hampering search operations, raising fears evidence from the debris could be tampered with.

According to the head of Ukraine's emergency services, the rebel fighters who Kiev says shot down the passenger plane, are restricting the search and emergency services efforts.

"The search work is difficult because we are talking about a big radius ... but also because armed terrorists who are on the spot are hampering things," Serhiy Bochkovsky, an emergency service official, said, according to Reuters.

Earlier, Ukrainian President Petro Poroshenko had said a "terrorist attack" brought down Malaysian Airlines passenger plane with 298 people on board.

However, Russian President Vladimir Putin said Ukraine, which is fighting off a vibrant pro-Russian insurgency in the eastern parts of the country, was responsible for the shooting down of the Malaysian passenger plane.

US President Barack Obama warned against possible attempts to tamper with the debris from the crash.

"The president emphasised that all evidence from the crash site must remain in place on the territory of Ukraine until international investigators are able to examine all aspects of the tragedy," the White House said in a statement.

Meanwhile, Ukraine has said it has got evidence that incriminates pro-Russian separatists. Phone calls of alleged pro-Russian fighters intercepted by authorities in Kiev, appear to show separatists had fired the missile that shot down MH17.

The Malaysia Airlines Boeing 777 plane was shot down over the town of Shaktarsk in eastern Ukrainian region of Donetsk while it was flying from Amsterdam to Kuala Lumpur carrying 298 passengers.

Malaysian Prime Minister Najib Razak has clarified that the jetliner's flight path was not a restricted one.

"The aircraft's flight route was declared safe by the International Civil Aviation Organisation," Najib said.

The International Air Transportation Association has also endorsed the Malaysian authorities' statement.

US Secretary of State John Kerry, who said he was "horrified by the crash of Malaysia Airlines flight MH17," promised to assist in an international investigation.

"The United States government remains prepared to assist with a credible, international investigation any way we can, and we will continue to be in touch with all relevant partners as we seek the facts of what happened today."

Meanwhile, Russia's Interfax news agency reported that Ukrainian rebel forces found the black box from Malaysian Airlines MH17 Boeing 777 aircraft.