Ghana's defence and environment ministers were killed in a military helicopter crash on Wednesday, the presidency said, after the air force chopper carrying three crew and five passengers came down in a forest in the south.

Television station Joy News broadcast cell phone footage from the crash scene showing smouldering wreckage in a heavily forested area earlier in the day, before it was revealed that ministers Edward Omane Boamah and Ibrahim Murtala Muhammed were among the dead.

Boamah became President John Mahama's defence minister shortly after Mahama's swearing-in in January.

Muhammed, 50, was serving as the minister of environment, science and technology.

He had been scheduled to attend the UN talks currently underway in Geneva aimed at hammering out a landmark global treaty on combating the scourge of plastic pollution.

Ghanaian media reported that the helicopter was on its way to an event on illegal mining -- a major environmental issue in the west African country.

Everyone on board was killed in the accident in the southern Ashanti region, authorities said.

"The president and government extend our condolences and sympathies to the families of our comrades and the servicemen who died in service to the country," said Mahama's chief of staff Julius Debrah.

The Ghanaian Armed Forces said investigations had been launched to determine the cause of the crash of the Z9 helicopter.

The military had reported earlier Wednesday that an air force helicopter had dropped off the radar after taking off from Accra just after 9am local time. It had been headed towards the town of Obuasi, northwest of the capital.

Alhaji Muniru Mohammed, Ghana's deputy national security coordinator and former agriculture minister, was also among the dead, along with Samuel Sarpong, vice chairman of Mahama's National Democratic Congress party. (AFP)