The announcement came after visiting South Korean President Moon Jae-in and Abu Dhabi Crown Prince Mohammed bin Zayed toured the $20-billion Barakah plant, the state-run WAM news agency reported.

The plant west of Abu Dhabi is being constructed by a consortium led by the Korea Electric Power Corporation (KEPCO).

Sheikh Mohammed described the achievement as "historical" for the energy sector of UAE, which is rich in oil and gas but is looking to increase other sources of power.

The UAE previously announced the first reactor would start operations in 2017 before delaying the start date.

The Emirates Nuclear Energy Corporation, which is overseeing the nuclear programme, is awaiting the approvals from the regulatory authority to start operating.

Nuclear and renewables, which currently make up a small portion of the federation's energy sources, are slated to contribute around 27 percent to UAE electricity needs by 2021.

The second reactor is 92 percent complete, the third 81 percent while 66 percent of the fourth has been completed, WAM reported.

When fully operational, the four reactors will produce 5,600 megawatts of electricity, or around 25 percent of the country's needs, according to the UAE energy ministry.

UAE says it aims to continue diversifying toward its goal of 50-percent clean energy by 2050.

Saudi Arabia, the world's top crude oil exporter, also has accelerated plans to acquire nuclear energy and is expected to prequalify several international companies next month to bid for building two nuclear reactors.