Defending champions Pohang had seen their lead at the top cut to two points after drawing 0-0 at Jeju United when the league resumed on Saturday after a mid-season break.
For the second match running, Hwang Sun-hong's men drew a blank in a stalemate with Seoul at the Steelyard Stadium.
That draw meant second-placed Jeonbuk Motors - who beat Busan I'Park on Saturday - had the chance to move level on points with Pohang.
However, they missed that opportunity after being held to a 1-1 by Jeju United, who could have gone second with a win.
Indeed, it appeared as though the visitors would leapfrog Jeonbuk after Song Jin-hyung opened the scoring six minutes before half-time.
Jeonbuk took a share of the spoils, however, when Kaio bagged his third goal of the campaign in the 79th minute.
Jeonnam subsequently moved to within three points of top spot in a 3-1 victory against Gyeongnam at the Gwangyang Stadium.
Kim In-han gave the visitors hope of snapping a nine-match winless run in the league, but Ahn Yong-woo and a goal in each half from Lee Jong-ho curtailed those hopes.
Another team to benefit were Suwon Bluewings, who climbed to fourth courtesy of a battling 3-2 win at home to Ulsan, despite a brace from Lee Yong for the visitors.
Elsewhere, rock-bottom Incheon United were pegged back in a 1-1 draw with Seongnam, while Sangju Sangmu defeated Busan I'Park 2-0.
