Fossil fuels will be vital for the world's energy for decades to come and must be made cleaner and more efficient, Saudi Arabian Oil Minister Ali al-Naimi said on Monday.

Given their massive scale, non-renewables will remain the world's energy workhorse for many decades to come, he told an energy and environment conference.

While the days of easy oil may be over, the days of oil as a primary fuel source for the people of the world are far from over, said Naimi, speaking for the world's top oil exporter.

He said Saudi Arabia was investing in solar power and hoped to become the world's leading supplier.

The minister had attended OPEC's Sunday meeting in Vienna, which had agreed to hold oil output steady. Naimi reiterated the group's view that investment in future energy projects was under threat due to the world economic downturn and low oil prices.