Ukrainian investigators said they had gained access to the black boxes that may reveal whether an airliner that crashed in Tehran was shot down by an Iranian missile as fears rose that the crash site had been tampered with.

Vadim Prysatsko, the Ukrainian foreign minister, said on Friday that Iran had allowed a 50-strong Ukrainian team access to the black boxes and the crash site, but refused to comment on Western governments' claims that it was hit by anti-aircraft fire.

"We're analyzing pieces of the plane, we're analyzing the bodies and we're analyzing chemical residue," Mr Prystaiko said at a press conference in Kyiv. "We want to come to conclusions. We don't want to come to them right now. Right now, we're not ruling out any theories."

Ukraine International Airlines flight PS752 crashed six minutes after taking off from Tehran’s Imam Khomeini International Airport early on Wednesday morning, killing all 176 people on board.