John Kerry meets Oman's Sultan Qaboos in Germany

US Secretary of State John Kerry flew into Munich on Saturday and met with Oman's ailing Sultan Qaboos.

State Department spokesperson Jen Psaki said the purpose of the private meeting was "to express his gratitude for their longstanding and strong relationship."

The sultan has not returned to Oman since travelling overseas for medical tests six months ago for suspected cancer, stirring fears over stability in his Gulf country.

Kerry was only accompanied by one aide for the private talks, with the small number of reporters travelling with the top US diplomat en route for India barred from access or from photographing the event.

The 74-year-old absolute Omani ruler is not only sultan, but also prime minister, as well as holding the foreign affairs, finance, defence and interior portfolios.

But without children, or even brothers, he has no direct heir, and his absence is raising questions over who will succeed him.

Qaboos, who has been on the throne for 44 years, travelled to Germany in July for medical tests, with one diplomat saying he is suffering from colon cancer.

Since then, he has made only one appearance, in a brief video that was broadcast on national television on 5 November.

In it, a visibly weakened Qaboos spoke of what he called the "good results" of his treatment, but added that his condition requires monitoring.

Oman played a key role in bringing Tehran back to the negotiating table over its suspect nuclear program, by hosting secret talks between the US and Iran in 2012 and 2013.

Muscat has close links with Tehran and has often been a conduit for the US which has no diplomatic relations with Iran.

Kerry last met with the sultan in May 2013 during an official trip to Muscat.

He was only set to be on the ground in Munich for a few hours, before leaving again to head for Ahmadabad in western India where he will address a major trade and investment summit.