Japan, China and South Korea are planning to hold a trilateral leaders summit in Tokyo around Dec. 19 to 20, a diplomatic source said Thursday, with all eyes on whether South Korean President Park Geun-hye will be able to make the trip amid mounting domestic pressure for her to resign.

The talks were originally planned for late this month or early next month, but the prolonged Diet deliberations on the Trans-Pacific Partnership trade deal have prompted a postponement until after Russian President Vladimir Putin's Dec. 15-16 visit to Japan, the source said.

Park, who would be making her first visit to Japan since taking office in 2013, is a suspect in a widening investigation into allegations that her longtime confidante Choi Soon-sil influenced state affairs.