South Korea’s special prosecutor on Monday indicted former President Yoon Suk Yeol on additional charges of abusing power and aiding an enemy state related to his short-lived imposition of martial law last year.

Yoon attempted to provoke military conflict between South and North Korea in order to declare martial law, a prosecutor's spokesperson told a briefing, citing evidence found on a military official's mobile phone that included some words suggesting potential provocations against North Korea, such as "drones" and "surgical strike".

Yoon was removed from office by the Constitutional Court in April and is on trial for insurrection stemming from his failed martial law declaration. If found guilty, he could be sentenced to death.

Yoon has said consistently he never intended to impose military rule but declared martial law to sound the alarm on wrongdoings by opposition parties and to protect democracy from "anti-state" elements.

According to the memo, Yoon, former defence minister Kim Yong-hyun and former military intelligence chief Yeo In-hyung planned to induce a North Korean attack against the South, the spokesperson, Park Ji-young, said.

The trio conspired to create tensions in the country as a justification for Yoon to declare martial law, she said.

Kim and Yeo were also indicted on the same additional charges, the prosecutor said.

The special prosecutor's team has accused Yoon and his military commanders of ordering a covert drone operation into the North to inflame tensions between the neighbours and justify his martial law decree.

In October last year, North Korea said the South had sent drones to scatter anti-North Korean leaflets over Pyongyang, and published photos of the remains of a crashed South Korean military drone.

Despite intense scrutiny and political pressure, South Korea's military declined at the time to comment on the suspicion of conducting a drone operation. A defence ministry official on Monday said it had no comment on the matter.

Former defence minister Kim is also on trial on charges related to the martial law declaration.

Yeo has said he deeply regretted not challenging the order from Yoon, according to media reports. The prosecutor's spokesperson said Yeo was making excuses that did not make sense about the notes discovered on his mobile phone.