By highlighting battlefield gains in northern Syria by a reliable partner on the ground, the White House sought to further justify President Barack Obama's policy of limiting US military involvement in the fight to reverse Islamic State's conquests there and in neighboring Iraq.

But it was also a stark reminder of Washington's stumbling bid to turn moderate Syrian Arab rebels into a viable fighting force as well as a pointed message to the Iraqi government, which is struggling to rebuild its fragile military.

Kurdish YPG-led fighters captured a town held by Islamic State on Tuesday, inching closer to Raqqa, the hardcore Islamist group's de facto capital of a self-declared 'caliphate'.

The apparent blow to Islamic State's momentum there was achieved with the aid of stepped-up US air strikes and intelligence sharing, US officials said. The US military had air-dropped weapons to YPG fighters inside Syria during an earlier campaign on the Turkish border.

"This is, I think, an indication of how critically important it is for the United States to have a capable, willing and effective partner fighting ISIL on the ground," White House spokesman Josh Earnest said, referring to Islamic State by one of its alternative names.

But US officials, speaking on condition of anonymity, tempered that view, citing Islamic State's proven ability to adapt and quickly recover from setbacks. Kurdish forces, they said, may also need more time to consolidate their gains, "ISIL has proven a resilient force," a US Intelligence source said.

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