Prosecutors on Thursday questioned the chief of South Korea's state-run pension fund over allegations the presidential office influenced its decision to back Samsung's merger plan last year, as the investigation into the scandal involving President Park Geun-hye and her confidante widens.

Moon Hyung-pyo, chief of the National Pension Service (NPS), appeared before the Seoul Central District Prosecutors' Office to bear witness to suspicions it gave undue favors to the country's largest business group.

Prosecutors here are looking into whether the president pressured the state fund -- which held an 11.6-percent stake in Samsung C&T Corp. and a 5-percent stake in Cheil Industries Inc. -- to support Samsung in return for favors the conglomerate gave to the president's close friend Choi Soon-sil. Choi, currently in custody, was indicted Sunday on a string of corruption charges.

Moon is suspected of playing a role when the merger deal was struck last year as the country's health minister. He led the Ministry of Health and Welfare from December 2013 to August 2015, before becoming the head of the pension fund late last year.

"I am a little puzzled," Moon told reporters before entering the prosecutors' office. "I could not interfere in the merger process. It is what the investment management department in the NPS does and I have never given any opinions regarding it (the merger plan).

"I received reports (about the NPS' support for the plan) afterwards and I heard there were no procedural flaws" he added.

Moon also said he did not have any contact with the presidential office Cheong Wa Dae or Samsung beforehand.

Last year, then Samsung C&T, the group's construction arm, successfully merged with Cheil Industries, the then de facto holding firm of the conglomerate.

At the time, Samsung had faced a strong challenge from individual shareholders, led by US hedge fund Elliott Associates over the merger plan. The deal was widely seen as an attempt to smoothly transfer power from group owner Lee Kun-hee to his son, Jae-yong.

The NPS' backing for the merger drew criticism as its decision was not reviewed by an independent panel which advises the state fund in exercising voting rights for invested firms. (Yonhap)