A South Korean special prosecutor said it had summoned Samsung Group leader Jay Lee as a suspect in a scandal involving President Park Geun-hye.

Prosecutors have been checking whether Samsung's support for a business and foundations backed by Park's friend Choi Soon-sil was connected to a 2015 decision by the National Pension Service to back a controversial merger of two Samsung Group affiliates.

Samsung, South Korea's largest business group, has acknowledged making contributions to two foundations as well as a consulting firm linked to Choi.

The prosecution summoned two senior Samsung Group officials this week for questioning, though they were classified as witnesses.

READ MORE: Park in trouble over Choi Soon-sil links

At a December parliament hearing, the executive denied that the firm paid bribes to pave the way for the 2015 merger.

Samsung made the biggest contributions of 20 billion won ($16.8m) to Choi's foundations, followed by Hyundai, SK, LG and Lotte.

In December, investigators also summoned Kim Jae-youl, chief of the sports marketing unit of Samsung Group, as they look into allegations that the business giant sponsored the president's jailed friend, Choi, to receive government favours.

Samsung is separately accused of funnelling millions of dollars to Choi to bankroll her daughter's equestrian training in Germany.

President Park could become South Korea's first democratically elected leader to leave office early after parliament voted in December to impeach her over the corruption scandal, a decision that must be approved or overturned by the Constitutional Court.