Turkey wants Saudi authorities to tell them the whereabouts of the body of murdered journalist Jamal Khashoggi, who was killed last month in the Saudi consulate in Istanbul, the justice minister said on Thursday.

Turkey expected Saudi Arabia to co-operate in its investigation and there must be no cover-up, minister Abdulhamit Gul told reporters.

"We are looking for answers to the question of where the body is," he said.

Khashoggi wrote a column for the Washington Post that criticised the Saudi government and its de facto ruler Crown Prince Mohammed bin Salman.

The Saudi government initially denied Khashoggi had been killed then later said he died in an unplanned "rogue operation". Last week, the kingdom's public prosecutor Saud Al Mojeb said the attack was premeditated.

"This case cannot be covered up, and we are expecting close cooperation from Saudi authorities on the investigation we are conducting transparently and meticulously," Gul said.

Istanbul chief prosecutor Irfan Fidan said after talks with Mojeb on Monday and Tuesday that Khashoggi was suffocated in a premeditated killing as soon as he entered the consulate, and his body was then dismembered and disposed of.