Prosecutors have summoned Ulsan Mayor Song Cheol-ho as a suspect in their ongoing probe into the presidential office's alleged meddling in the 2018 Ulsan mayoral race.

The Seoul Central District Prosecutors' Office on Monday began questioning Song for the first time on whether he received support from the top office and the ruling camp in drawing up election pledges, including the construction of a public hospital.

Prosecutors suspect the top office helped Song, a confidant of President Moon Jae-in, draw up and implement his pledges and ordered police to investigate alleged corruption surrounding incumbent conservative mayor Kim Gi-hyeon ahead of the June 13, 2018 election.

Song and Ulsan Deputy Mayor Song Byeong-gi reportedly met with a senior presidential official in January 2018 to discuss the construction of a public hospital in Ulsan.

Prosecutors have carried out multiple raids at government offices, including police headquarters and the Finance Ministry, but they failed to search the presidential office after the top office refused cooperation.