President Barack Obama announced Monday the biggest expansion of U.S. ground troops in Syria since its civil war began, but the move was unlikely to mollify Arab allies angry over Washington's cautious approach to the conflict.

The deployment of up to 250 Special Forces soldiers increases U.S. forces in Syria roughly sixfold and is aimed at helping militia fighters who have clawed back territory from Islamic State group militants in a string of victories.

Defence experts said giving more fighters on the ground access to U.S. close air support could shift the momentum in Syria. But a senior member of the Saudi royal family who asked not to be identified dismissed the decision as "window dressing."