TOKYO (Reuters) - Crude oil prices rose by around 1 per cent on Thursday after Saudi Arabia and its Gulf Arab allies began a military operation in Yemen, including airstrikes.

The operation, against Houthi rebels who have driven the president from the country's capital, could stoke concerns about the security of oil shipments from the Middle East.

Analysts had earlier said they were worried a proxy war might break out on the Arabian peninsula, home to the world's biggest oil fields, if the conflict drew in Saudi Arabia and rival Iran.

Brent futures for May delivery had climbed just over 1 per cent, or 62 cents, to US$57.10 a barrel by 0106 GMT, after rising as high as US$57.25 in earlier trade.

U.S. crude for May delivery was up 50 cents at US$49.72 a barrel, after trading as high as US$49.90.

Saudi Arabia announced on Wednesday it had launched military operations in Yemen, carrying out air strikes in coordination with a 10-country coalition seeking to beat back Houthi militia forces besieging the southern city of Aden where President Abd-Rabbu Mansour Hadi had taken refuge.