WASHINGTON (Reuters) - Saudi Arabia's ambassador to the United States, Prince Turki al-Faisal, abruptly resigned his post after 15 months on the job and left the country, The Washington Post reported on Tuesday, citing U.S. officials and foreign envoys.

Al-Faisal told his staff he was leaving to spend more time with his family, the newspaper reported, citing Arab diplomats. His predecessor, Prince Bandar bin Sultan, held the job for 22 years.

A Saudi Embassy spokesman said he could not confirm the report.

Saudi Arabia has been a key ally to the United States and is the world's top oil exporter.

Diplomats in Riyadh noted in recent days that the health of his brother, Saudi Foreign Minister Prince Saud al-Faisal, was not good and he appeared slow or ill in recent public appearances.

That has fueled rumors that Turki would succeed his ailing brother in the foreign minister position, The Post reported.

The departure also came days after Turki fired a consultant who wrote an opinion piece published in The Washington Post that suggested the Saudi kingdom would back Iraq's Muslim Sunnis in the event of a wider sectarian conflict.

The article by a Saudi government security adviser, Nawaf Obaid, said the kingdom would intervene with funding and weaponry to prevent Shiite militias from attacking Iraq's Sunnis and suggested Saudi Arabia could bring down world oil prices to squeeze Shiite power Iran.