Oil settled lower on Tuesday after rising to the highest since July 2015 the previous day, while tension flared between Saudi Arabia and Iran, and the Saudi crown prince tightened his grip on power. Brent crude futures settled down 58 cents, or 0.9 percent, at $63.69 a barrel, having climbed 3.5 percent on Monday. U.S. West Texas Intermediate (WTI) crude fell 15 cents, or 0.3 percent, to $57.20 a barrel.

Saudi Crown Prince Mohammed bin Salman moved to shore up his power base with the arrest of royals, ministers and investors, which an official described as part of "phase one" of a crackdown. More tellingly, tensions escalated between OPEC members Saudi Arabia and Iran, which analysts said did more to rattle the market than the prince's purge.

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