MOSCOW, July 29 (Reuters) - The Russian rouble weakened against the dollar on Monday, the corporate tax payment day, losing support from accelerated foreign currency sales last week by exporting companies which had accumulated roubles to pay taxes.

Trading in major currencies shifted to the over-the-counter market, obscuring the pricing data, after Western sanctions on the Moscow Exchange and its clearing agent, the National Clearing Centre, were introduced on July 12.

The rouble received some support from the central bank's decision to raise its key interest rate by 200 basis points to 18% last Friday.

Against the yuan, which had already become the most traded foreign currency in Moscow before the latest sanctions were imposed, the rouble shed 0.1% to 11.56, according to an analysis of the OTC market.

It was down 0.3% at 93.72 against the euro.

Brent crude oil, a global benchmark for Russia's main export, was up 0.1% at $81.23 a barrel on fears of a widening conflict in the Middle East. (Reporting by Gleb Bryanski; Editing by David Holmes)