By Angus McDowall

RIYADH (Reuters) - Saudi Arabian intelligence chief Prince Bandar bin Sultan, the architect of Riyadh's attempts to bring down Syrian President Bashar al-Assad, has been removed from his post, state media reported on Tuesday.

His departure, months after he was quoted warning of a "major shift" from the United States over its Middle East policy, may help to smooth relations with Washington as Riyadh pushes for more U.S. support for Syrian rebels.

Prince Bandar, who has recently spent time in the United States and Morocco for medical treatment, was replaced on an interim basis by a deputy.

"Prince Bandar was relieved of his post at his own request and General Youssef al-Idrissi was asked to carry out the duties of the head of general intelligence," state news agency SPA said, citing a royal decree.

The decree did not say if Prince Bandar would continue in his other position as head of the National Security Council.

A former ambassador to the United States, Prince Bandar was appointed intelligence chief in July 2012, in charge of helping Syrian rebels bring down Assad, an ally of Riyadh's biggest regional rival Iran.

He was also closely involved in Saudi support for Egypt's military rulers after they ousted Islamist president Mohamed Mursi last year, diplomatic sources in the Gulf have said.

COMPLICATED COOPERATION

However, despite his longstanding connections in Washington and personal relations with world leaders stretching back decades, Prince Bandar proved a sometimes abrasive figure in his efforts to corral Western support for Syrian rebels.

Western officials have said in private that his comment in October about a "major shift" from the U.S. following President Barack Obama's decision not to use military strikes against Assad had complicated cooperation on Syria.

A trip to Moscow last year to push President Vladimir Putin to abandon his support for Assad also failed to produce results.

"He had been more or less disengaged from the Syrian file for the past five months. The responsibility was divided between a number of people - officers in the intelligence sphere and other princes. So the reality is that any changes have already happened," said Mustafa Alani, a security expert with close ties to Riyadh.

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Saudi support for the rebels, including arms supplies, training and financing, has been hampered by infighting between opposition groups and difficulties in working out which of them pursued militant ideologies that could endanger Riyadh.

Earlier this year, Riyadh recalibrated its Syria policy to focus more on preventing militant groups there from posing an eventual threat inside the kingdom, something that was pushed by powerful Interior Minister Prince Mohammed bin Nayef.

(Additional reporting by Reem Shamseddine; Editing by Sami Aboudi and Robin Pomeroy)