By Nayera Abdallah and Pesha Magid

DUBAI/RIYADH (Reuters) -Nadhmi al-Nasr, the long-time chief executive of the $500 billion NEOM mega-project at the heart of Saudi Arabia's drive to diversify away from oil, has departed, NEOM said on Tuesday without giving a reason.

Crown Prince Mohammed bin Salman, known as MbS, has poured hundreds of billions of dollars into development projects through the kingdom's PIF sovereign wealth fund.

NEOM, a Red Sea urban and industrial development nearly the size of Belgium due to house nearly nine million people, is central to the prince's Vision 2030 plan to create new engines of economic growth beyond oil.

But some of the schemes have had to be scaled back due to rising costs, including The Line, a futuristic city between mirrored walls extending 170 km (106 miles) into the desert within NEOM.

Reuters reported in May that the $925 billion PIF was weighing a reorganisation, aiming to sharpen its focus on investments that have a higher chance of success.

Saudi Arabia, the world's top oil exporter, is still heavily reliant on hydrocarbon revenues, and low oil prices and production have hit state coffers.

A source with knowledge of the matter told Reuters that part of the reason for Al-Nasr's departure was that he was not delivering on key performance indicators, though the source did not specify which goals were not met.

NEOM did not immediately respond to a request for comment on the reasons for Al-Nasr's departure.

Project leaders have been working under extremely tight deadlines to deliver massive developments by the 2030 deadline with several schemes falling behind schedule or facing delays.

Aiman al-Mudaifer was named as NEOM's acting CEO. He has been the head of the PIF's Local Real Estate Division since 2018 and has a deep understanding of NEOM and its projects, NEOM said.

"As NEOM enters a new phase of delivery, this new leadership will ensure operational continuity, agility and efficiency to match the overall vision and objectives of the project," it said.

In his role at PIF, Al-Mudaifer oversees all local real estate investments and infrastructure projects, and he is a board member of several prominent companies in the kingdom.