Yoon made a defiant address earlier Thursday in which he defended his shock decision to declare martial law and send troops to the South Korean parliament last week, plunging the democratic nation into political uncertainty.

The embattled leader also lashed out at China during his speech, accusing Chinese nationals of photographing and filming military sites in South Korea in apparent attempts at espionage.

Beijing's foreign ministry hit back later Thursday, saying Yoon was "groundlessly hyping up so-called Chinese espionage".

"China... is deeply surprised and dissatisfied with these remarks made by the South Korean side," ministry spokeswoman Mao Ning said at a regular press briefing, without referring to Yoon by name.

"This is not conducive to the healthy and stable development of China-South Korea relations," she added.

During his address, Yoon accused the main opposition party of blocking his efforts to amend South Korea's anti-spying law.

He said current legislation left "no way to punish foreign nationals for espionage" and cited two cases of alleged spying by Chinese citizens.

They included three people caught flying a drone and filming a US aircraft carrier in the southern port city of Busan, whose devices also contained photos of military installations.

In the other case, Yoon said, a man in his 40s travelled from China to Seoul and "made a beeline" to film South Korea's National Intelligence Service with a drone.

Yoon did not give further details of either case and spent the majority of his speech arguing the opposition had caused a "national crisis", pledging to "fight with the people until the very last minute".

The 63-year-old, who apologised again for making people "surprised and anxious" with his martial law declaration, is banned from leaving the country, faces an impeachment vote on Saturday, and has been assailed by protests calling for his resignation.

Mao said Thursday that Beijing "firmly opposes the South Korean side's linking internal issues with China-related factors".

The two countries are major trade partners but have long been out of step politically.

China's ruling Communist Party is the main economic and political backer of North Korea, with which Seoul remains technically at war.

South Korea, meanwhile, is a key US ally and hosts a strong American military presence, while Beijing's ties with Washington have soured in recent years.