The King has greeted four British veterans of the Korean War ahead of a palace reception to mark the 70th anniversary of the conflict.
Charles was pictured standing in the 18th Century Room as he held the audience at Buckingham Palace on Tuesday.
He met Alan Guy, Mike Mogridge, Brian Parritt and Ron Yardley and can be seen smiling and sitting with the men.
Later, the Princess Royal and the Duchess of Edinburgh will host a reception for around 200 Korean War veterans in the Bow Room.
Anne will deliver a speech on behalf of the King, in which he reflects on what was once called “the Forgotten War”.
The conflict ended with the signing of an armistice on July 27 1953 and the anniversary was acknowledged during the Korean state visit to the UK in November.
British and Commonwealth forces were deployed following a United Nations call to support South Korea after the invasion by its northern neighbour.
Of the around 60,000 members of the British Armed Forces who served, more than 1,100 were killed and 2,500 were wounded, according to the National Army Museum.
Charles is continuing with his head of state duties, including audiences and desk work, while undergoing treatment for an undisclosed form of cancer.
Last Wednesday, he welcomed Baroness Scotland, the Commonwealth secretary-general, to Buckingham Palace for a private meeting.
He also made a virtual appearance at the Commonwealth Day service last Monday in a pre-recorded video message.
In his annual address to the family of nations, he pledged to continue to serve the Commonwealth “to the best of my ability”.
Since his diagnosis was announced on February 5, the King has postponed all public-facing engagements.
His last public appearance was on February 11, when he and the Queen waved to well-wishers as they walked to church in Sandringham.
The royal family have had a challenging start to 2024, with the Princess of Wales currently not carrying out official duties following abdominal surgery on January 16.
Her absence from public life has led to wild conspiracy theories on social media about her health.
Kensington Palace have not disclosed details of her condition but previously said it was not cancer-related.
On Saturday, Kate was filmed smiling and chatting with her husband, the Prince of Wales, during a visit to a farm shop in Windsor.
The video, obtained by The Sun, suggests her recovery from surgery has progressed.
The royal couple also spent Sunday morning watching Prince George, Princess Charlotte and Prince Louis taking part in a sporting event, according to The Sun.