Angela Merkel (R) and Denmark PM Mette Frederiksen listen to the national anthems. (AFP)

Angela Merkel remained seated during national anthems at an official ceremony on Thursday, as the German chancellor apparently sought to prevent a repeat of uncontrollable shaking with a rare change of protocol.

With questions swirling about Merkel's health after she suffered three episodes of shaking in public in less than a month, the veteran leader opted to play safe.

After greeting Denmark's new Prime Minister Mette Frederiksen at the portico of the chancellery, a smiling Merkel walked her over to a podium where both leaders took their seats.

The unusual move came a day after a similar ceremony when the German chancellor was seen shaking involuntarily for the third time.

On Wednesday, Merkel began trembling as national anthems were being played at the reception of Finnish Prime Minister Antti Rinne.

Just over an hour later, she attended a press conference as planned and told journalists that her health was no cause for concern.

She explained that she was simply still in a phase of "processing" a previous shaking spell, but that "there has been progress".

"I will have to live with it for a while," added Merkel, who turns 65 next week.

"Just like how it has come, one day it will go away too," she said.

More transparency?

The shaking on Wednesday was visible although less severe than during the first episode in June.

On that occasion she appeared unsteady and shook as she stood in the midday sun next to Ukrainian President Volodymyr Zelensky, whom she was welcoming with military honours.

That first bout of shaking was blamed on dehydration. But a second episode struck a week later at the end of June, just hours before she was due to board a plane for a G20 summit in Japan.

Officials have sought to play down the repeated shaking, insisting that Merkel is well while refusing to address detailed questions about her medical condition.

But several media outlets said Merkel would need to provide more transparency if she wanted the speculation to go away.

The daily Sueddeutsche Zeitung said Merkel's explanation belies "her fear that the public could doubt her strength and ability, especially if she admitted that she has allowed herself to be examined. Meaning: she herself has doubts".

It said she should state clearly whether she has undergone medical tests to get to the bottom of the mysterious trembling spells.

"It's about sending a signal that she is taking her own health seriously."

Merkel has been leader of Europe's biggest economy for almost 14 years.

Frequently called the European Union's most influential leader and the most powerful woman in the world, Merkel has said she will leave politics at the end of her term, in 2021.