The downing of a Malaysia Airlines jet over rebel-held eastern Ukraine has raised questions over why the company persisted in flying in conflict-zone airspace that many other Asian carriers had abandoned months ago.

The air corridor over Ukraine has always been a crowded one for flights between Europe and Asia -- particularly Southeast Asia -- and re-routing around the airspace would mean an increase in flight time and fuel costs.

British Prime Minister David Cameron said Friday that if it is confirmed a Malaysian airliner was shot down over Ukraine, those responsible "must be brought to account."

Malaysia Airlines flight MH17 was flying from Amsterdam to Kuala Lumpur when it crashed on Thursday, killing all 298 people on board including nine Britons.

Russia warned Friday it may respond if cross-border shooting from Ukraine continues, sharply raising tensions over the crisis a day after a Malaysian jet crashed in the country's rebel-held east killing all 298 people aboard.

"We already warned that if this continues then we will take measures. At least if it is clear that this has been done deliberately I am convinced that such a firing position should be neutralized as a one-off measure," Russian Foreign Minister Sergei Lavrov told Rossiya 24 state television.

A top separatist leader in east Ukraine on Friday ruled out a temporary truce with government forces but pledged to allow investigators to access the site of downed Malaysia Airlines jet MH17.

"There is no question of a ceasefire but we will let experts access the site of the catastrophe," Alexander Borodai, the prime minister of the self-proclaimed "Donetsk People's Republic," told journalists.

Two Lebanese nationals, holding the Australian citizenship, were among the 298 people on board the Malaysian jetliner that was shot down over Ukraine.

According to Voice of Lebanon radio (100.5), Albert and Mary Rizk are among the victims who died along with dozens of others in the tragedy on Thursday.

Glen Thomas, a spokesman for the World Health Organisation, was among the 298 people killed in the crash in eastern Ukraine of a Malaysia Airlines jet, the U.N. agency said Friday.

Thomas was headed to Melbourne for a global AIDS conference along with dozens of others on the flight that ended in tragedy on Thursday.

Russia's state-controlled television on Friday wheeled out conspiracy theories to explain the Malaysian air crash including one holding that it was a bungled attempt to down President Vladimir Putin's presidential jet.

After Putin said that Ukraine was responsible for the crash, Russian state television focused on several theories that pinned the blame accordingly. 

Rescue workers at the crash site of Malaysian airliner MH17 in eastern Ukraine told an Agence France Presse reporter Friday that they had found one of the black boxes from the passenger liner.

Emergency crews were working through the debris of the downed jet that was spread out across an area stretching for kilometers while rebels controlling the area have pledged to allow international investigators access to the site.

The world AIDS community Friday mourned several passengers on a crashed Malaysian Airlines plane who were heading to Australia for a global conference on the epidemic.

President Barack Obama said Thursday the crash of a Malaysian airliner in eastern Ukraine was a "terrible tragedy" and said U.S. officials were trying to establish if any Americans were on board, as the UK expressed “shock” and France urged an immediate probe.

"The world is watching reports of a downed passenger jet near the Russia/Ukraine border. And it looks like it may be a terrible tragedy," Obama said, before going ahead with a previously scheduled event in Wilmington, Delaware.