The killing of journalist Jamal Khashoggi was planned days in advance, Turkish President Recep Tayyip Erdogan has told MPs from his ruling party.

He said Turkey had strong evidence Khashoggi was killed in a premeditated and "savage" murder at the Saudi consulate in Istanbul on 2 October.

Fifteen Saudi nationals, Mr Erdogan said, arrived in Istanbul in three teams, on separate flights, in the days and hours leading up to the murder.

A day before the killing, he said, some members from the group travelled to Belgrad forest, near the consulate - an area searched last week by Turkish police looking for the body.

He also described how the team had removed the hard drives from the consulate's surveillance camera system prior to the arrival of Khashoggi, who was visiting to obtain documents for his forthcoming marriage.

A man who looked like Khashoggi, wearing his outfit, glasses and a fake beard, was among a group who left the consulate the same day as the killing, the president added.

On Monday, CNN broadcast images appearing to show that Saudi operative leaving the consulate.

President Erdogan confirmed 18 people had been arrested in Saudi Arabia over the case.

He told MPs from his ruling AK party the 18 should "be tried in Istanbul", adding that "all those who played a role in the murder" would be punished.

He also called for the suspects to be tried in Istanbul.

He demanded Saudi Arabia provide answers about where Khashoggi's body was, and who had ordered the operation.

The Saudi kingdom has provided conflicting accounts of what happened to Khashoggi, a US resident and Washington Post contributor. After weeks of maintaining he was still alive, the authorities now say the 59-year-old was killed in a rogue operation.

Mr Erdogan's address coincided with the start of an investment conference in Saudi Arabia that has been overshadowed by the Khashoggi case. Dozens of government and business leaders have pulled out, but Crown Prince Mohammed bin Salman appeared at the event on Tuesday.

Many world leaders have condemned the murder of the prominent Saudi critic and demanded a full investigation. (BBC)