Japanese prosecutors raided offices belonging to the ruling party on Tuesday over a funding scandal that forced Prime Minister Fumio Kishida to replace four ministers last week.
Public broadcaster NHK aired footage of investigators entering a building that houses the office of the largest faction of the ruling Liberal Democratic Party (LDP), after reports that officials took kickbacks of 500 million yen ($3.4 million).
"The party must work to restore public confidence with a strong sense of urgency," Kishida told reporters, adding that he could not talk about the investigation in detail.
"Investigations must be carried out rigorously," he added.
Earlier, party number-two Toshimitsu Motegi described the raids as "extremely regrettable", telling reporters that the party would take "necessary measures while observing the fate of the investigation".
Kishida last week sacked four ministers, including the top government spokesman and the trade minister, over failure to report political funds as required by law.
All four ministers were from the LDP faction previously headed by ex-premier Shinzo Abe, who was assassinated last year, after leaving office.
Tokyo public prosecutors also raided the office of another LDP faction on Tuesday, but two members of the faction remain as ministers.
The prosecutors have reportedly been probing five of the six LDP factions over alleged unreported political funds by interviewing officials in charge of factions' accounts.
Kishida's poll ratings are the worst for any premier since the LDP returned to power in 2012, dragged down by voter anger about inflation, as well as his handling of a string of earlier scandals.
The latest poll published by the Jiji news agency on Thursday showed public support for Kishida's cabinet at just 17.1 percent, down 4.2 percentage points.