DOHA - The emir of Qatar, a key U.S. ally, named his younger son as heir apparent on Tuesday, replacing his other son Sheikh Jassim bin Hamad bin Khalifa al-Thani, who diplomats say has been asking for wider powers.

An official decree carried by the news agency QNA said Emir Sheikh Hamad bin Khalifa al-Thani accepted the decision by Sheikh Jassim to step down as crown prince and replaced him with his younger brother Sheikh Tamim bin Hamad bin Khalifa al-Thani.

Both sons are in their 20s, Reuters said.

There was no immediate explanation for the sudden change, but diplomats say that Sheikh Jassim had been complaining of being sidelined in the running of the oil-and-gas rich country.

QNA said Sheikh Jassim sent a letter to his father explaining his decision to step down, saying "the time is appropriate to step down and prepare for a successor".

The letter, carried by agency, did not give specific reasons but Sheikh Jassim said he had first accepted the post in 1996 to help his country after authorities foiled a coup plot against his father and arrested dozens of people.

Sheikh Jassim is Sheikh Hamad's third son. Sheikh Tamim, who heads Qatar's Olympic committee, is his fourth.

Diplomats told Reuters that Sheikh Jassim, who has held the post since 1996, was frustrated because he did not have enough official responsibilities. Another diplomat said the change might be due to health reasons, but did not elaborate.

Qatar, a small state with a history of palace coups, hosted the advance headquarters of U.S. forces in the war that toppled Iraqi President Saddam Hussein in April. The OPEC member has the world's third largest natural gas reserves.

Sheikh Hamad, 54, ousted his father, Sheikh Khalifa bin Hamad al-Thani, in 1995 in a bloodless palace coup while the old emir was in Switzerland.