The post of ambassador is key for the US and Saudi Arabia

Saudi officials said he wanted to spend more time with his family.

There are suggestions that he may be promoted to foreign minister, or that he resigned over differences with officials in Riyadh.

The post of ambassador to Washington is one of the most important for Saudi foreign relations.

In recent years, the US-Saudi relations have occasionally been uneasy.

Saudi Arabia opposed the US-led invasion of Iraq, and has been dismayed by events across its northern border since.

US-Saudi relations became strained after it was revealed that most of the hijackers involved in the 11 September attacks came from Saudi Arabia.

Theories

BBC Middle East analyst Roger Hardy says there are two possible reasons for his sudden resignation.

His elder brother, Prince Saud al-Faisal - his country's foreign minister for over 30 years - has been unwell for some time, and there's speculation that Prince Turki might succeed him.

However, a different and perhaps more plausible theory is that, as ambassador in Washington, Prince Turki felt he had not been getting sufficient support back home in Riyadh.

In particular, our correspondent says, it has been reported that he resented continuing interference from his predecessor in the post, Prince Bandar bin Sultan.

The two men could scarcely be more different - Prince Bandar is worldly, extrovert and brash, while Prince Turki is urbane, courtly and low-key.

Whatever the reasons behind it, Prince Turki's departure leaves a void in US-Saudi relations at a moment when the Middle East faces crises on several fronts, our correspondent says.