By Phil Stewart

WASHINGTON (Reuters) - The United States has carried out a fresh delivery of ammunition to fighters from the Syrian Arab Coalition battling Islamic State in northern Syria, pushing ahead with a strategy that initially unnerved ally Turkey, a U.S. official told Reuters on Sunday.

The delivery of ammunition represented only the second time the United States has moved to arm the Syrian Arab Coalition, a collection of about 10-12 groups numbering about 5,000 fighters. They are working with Kurds and others to claw back land from Islamic State.

In a shift in approach, the latest U.S. resupply operation was completed on Saturday by delivering the weaponry by land, the U.S. official said, speaking on condition of anonymity.

It was not immediately clear who transported the ammunition into Syria, but the official said American troops did not drive them into the country.

Further details on the operation were not immediately available.

The first U.S. delivery of ammunition to the group on Oct. 11 was carried out through an air drop. That triggered questions about how the United States could be certain the weaponry reached the right people, despite reassurances from the Pentagon.

NATO ally Turkey, which is wary of further advances by Syrian Kurdish YPG militia, registered its unease after the first air drop. It called in the U.S. ambassador to express concern that Washington's arms drops were aiding Syrian Kurdish militias.

Turkey, which has its own large Kurdish minority, objects to empowering those militias, although they have been the most effective U.S.-allied force against Islamic State in Syria.

The U.S. official said the United States was certain that the latest delivery reached the intended individuals.

The U.S. military declined comment on the operation. Captain Jeff Davis, a Pentagon spokesman, said only that the United States had signaled its intent to support Syrian Arabs battling Islamic State, and continued to do so.

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Washington's strategy in Syria has shifted from trying to train fighters outside the country to supplying groups headed by U.S.-vetted commanders.

The United States has also announced it would send dozens of U.S. special operations forces in the coming weeks to northern Syria to advise those opposition fighters combating Islamic State.

The U.S. military, when it carried out its first air drop of ammunition last month to the Syrian Arab Coalition, said it was going to make sure the weaponry was used correctly before providing additional arms.

The U.S. military later expressed confidence that the ammunition helped the Syrian Arab Coalition as they and other forces branding themselves collectively as the Democratic Forces of Syria took back a big swathe of territory around the village of al-Haul.

The U.S. official said the new delivery of weapons would help the fighters push further south into Islamic State-held territory.

(Reporting by Phil Stewart; Editing by Leslie Adler)