The population of Seoul, the capital of South Korea, is likely to gradually contract to 9.69 million by 2025 on a declining birthrate and aging citizens, a government forecast showed Monday.

South Korea, which is one of the world's fastest aging societies, also has the world's lowest birth rate with the average number of children born to a woman over her life time falling to 1.08.

Seoul's population is likely to stand at 10.03 million in 2010 and fall to 9.89 million in 2020, according to the forecast by Statistics Korea, the nation's statistics agency.

The agency also expected Pyongyang, the capital of North Korea, to become the second-most populous city on the Korean Peninsula in 2015, replacing Busan, the southern port city of South Korea.

Pyongyang's population may rise to 3.43 million in 2015, while Busan's population may shrink to 3.32 million by then, compared with an estimated 3.44 million next year, the agency said.