King Charles III experienced unspecified side effects of his cancer treatment and spent some time in the hospital on Thursday, Buckingham Palace said.

"Following scheduled and ongoing medical treatment for cancer this morning, The King experienced temporary side effects that required a short period of observation in hospital," Buckingham Palace said in a statement.

No details were provided on the length of the observation period or what the king's side effects entailed. He has since returned to Clarence House, one of the royal residences in London.

The monarch's engagements for Friday have been postponed as a result. Charles was scheduled to have four events in Birmingham that he is "greatly disappointed" to have to miss, according to the palace.

"He very much hopes that they can rescheduled in due course and offers his deepest apologies to all those who had worked so hard to make the planned visit possible," the palace said.

Buckingham Palace revealed that Charles had been diagnosed with cancer in February 2024, but information on the British royal's condition has been sparse. The announcement came weeks ahead of news that Princess Kate was undergoing "preventative chemotherapy."

The palace has not publicized what type of cancer either royal was diagnosed with nor the stage of the disease. Kate announced her cancer was in remission earlier this year.

A source told NBC News in December that Charles' treatment is "moving in a positive direction." The source said there's been progress in his condition but that treatment would continue into 2025 as the cancer is in a "managed condition."

Charles became king on Sept. 8, 2022, following the death of his mother, Queen Elizabeth II. She was the longest serving monarch in the United Kingdom's history and, subsequently, Charles was the longest-serving heir to the throne before becoming the sovereign.