People living within two kilometres of the site where the Antonov-12 came down have been warned to stay indoors.
The aircraft was flying from Serbia to Jordan when it crashed late on Saturday close to the city of Kavala, killing all eight people on board.
Eyewitness video showed the plane on fire and a huge fireball as it crashed.
Drones were used to survey the site of the wreckage out of caution, while Greek state-run TV reported the army, explosives experts and Greek Atomic Energy Commission staff were not approaching the site until it was deemed safe.
“The (air) measurements at the moment have not shown anything but nonetheless instability in the field was observed,” Lieutenant General Marios Apostolidis, of the Northern Greece Fire Brigade, told reporters.
“In other words, intense smoke and heat, as well as a white substance that we do not recognise, so a special armed forces team has to inform us what it is and whether we can enter the field.”
The pilot had reportedly requested an emergency landing at Kavala airport in Greece, due to an engine problem shortly after take off, but was unable to reach the runway.
The plane was noticed at about 22:45 local time (19:45 GMT) by residents.
Aimilia Tsaptanova – who saw the plane come down – said she was amazed it hadn’t crashed into their homes.
“It was full of smoke, it had a noise I can’t describe and it went over the mountain,” she said. “It passed the mountain and turned and crashed into the fields.
“There were flames, we were scared. A lot of cars came, but they couldn’t approach because there were continuous explosions.”
Serbia’s Defence Minister Nebojša Stefanovic said that the plane was transporting almost 11 tonnes (11,000 kg) of Serbian-made weapons to Bangladesh.
But there are conflicting reports of what kinds of weapons were on board.
Mr Stefanovic said it had included “illuminating mortar mines and training (mines)”, adding that the flight “had all necessary permissions in accordance with international regulations.”
A director from the arms dealer Valir also told the BBC there were landmines on board.
However, a spokesman for Bangladesh’s military public relations office told the BBC’s Bengali service that the plane contained mortar shells bought from Serbia for the training of army and border guards.
The plane, which was due to make stops in Jordan, Saudi Arabia and India before reaching its final destination of Dhaka, the capital of Bangladesh, was being operated by Meridian, a Ukrainian cargo airline. The crew are understood to have been Ukrainian.
