Turkey deepened on Monday its investigation into the death of Saudi dissident journalist Jamal Khashoggi in Riyadh's consulate in Istanbul.

Several prosecutors interrogated five witnesses, all of whom are consulate employees, on Monday, state news agency Anadolu reported.

More than 20 other witnesses, including Turks and foreigners, were set to be questioned over the course of the day.

Khashoggi, 59, was last seen on October 2 when he entered the Saudi consulate in Istanbul to seek marriage paperwork. The Saudi government admitted on Saturday that he died there, blaming a fistfight and saying 18 people were detained.

However, there has been widespread scepticism about the claim.

Turkish media reported that Khashoggi was murdered and dismembered based on recordings from the consulate. They say he was killed by a 15-member squad from Saudi Arabia.

Anadolu reported late Sunday that Turkish President Recep Tayyip Erdogan had held a call with US President Donald Trump in which the two leaders agreed to illuminate the case from all sides.

Erdogan has planned an announcement on Khashoggi's death for Tuesday.

French President Emmanuel Macron discussed the killing with his US counterpart Donald Trump, the Elysee Palace said on Monday.

The two leaders "shared their concerns over the circumstances that led to the tragic death of the Saudi journalist Jamal Khashoggi" during a call ahead of Trump's planned visit to France in November for the centenary of the armistice that ended World War I.

German Economy Minister Peter Altmaier on Monday called for Europe to stand together and unite on a position after the death of Khashoggi.

"Only if all European countries agree can we make an impact on the government in Riyadh," Altmaier told the German broadcaster ZDF.

Until the full details of what happened become clear, the German government announced on Sunday there would be no further weapons exports to Saudi Arabia.

"There will be no positive consequences if we're the only ones halting exports, and the other countries fill that gap," Altmaier said.

Altmaier would not comment on whether the exports that have already been approved would also be halted, saying that would need further consideration.

Annalena Baerbock, the national chairwoman of the Green Party, on Monday called for all exports, including those already approved, to be stopped immediately.

"That would really, really hurt Saudi Arabia," she said.

Independently, Indonesia on Monday called for a transparent investigation into the death of Khashoggi.

"Indonesia hopes that the investigation can be conducted transparently," Foreign Minister Retno Marsudi said after talks between visiting Saudi Foreign Minister Adel al-Jubeir and Indonesian President Joko Widodo.