The Syrian Observatory for Human Rights said air strikes in support of the ground operation killed 15 civilians

* U.S.-supported fighters target Islamic State

* Seek to drive IS from vital tract of Syrian land

* IS uses the turf to move fighters to Europe

* Small numbers of U.S. special forces will support push

* Offensive takes 16 villages - Observatory (Adds details on offensive, U.S., Turkish comments)

By Phil Stewart and John Davison

WASHINGTON/BEIRUT, June 1 (Reuters) - Thousands of U.S.-backed fighters opened a major new front in Syria's war, launching an offensive to drive Islamic State out of a swathe of northern Syria it uses as a logistics base, and were reported on Wednesday to be making swift progress.

The operation, which began on Tuesday after weeks of quiet preparations, aims to choke off the group's access to Syrian land along the Turkish border that the militants have long used to move foreign fighters back and forth to Europe.

"It's significant in that it's their last remaining funnel" to Europe, a U.S. military official told Reuters.

A small number of U.S. special operations forces will support the push on the ground to capture the "Manbij pocket" of territory, acting as advisers and staying back from the front lines, the officials said, speaking on condition of anonymity to discuss military planning.

"They'll be as close as they need to be for the (Syrian fighters) to complete the operation. But they will not engage in direct combat," the official said.

The operation will also count on airpower from the U.S.-led coalition, which pounded Islamic State positions near Manbij with 18 strikes on Tuesday, including six militant tactical units, two headquarters facilities and a training base.

ASSAULT

Driving Islamic State from its last remaining foothold at the Turkish border has been a top priority of the U.S.-led campaign against the group. The group controls around 80 km (50 miles) of the frontier stretching west from Jarablus.

Still, some U.S. military and intelligence officers caution Islamic State has proved adaptable, willing to change tactics.

In Iraq, for example, the Sunni extremist group has countered some territorial and other losses by staging attacks in Baghdad, the seat of the country's Shiite-led government.

The Syrian Observatory for Human Rights said U.S.-led air strikes in support of the ground operation killed 15 civilians including three children near Manbij in the last 24 hours. The Observatory's reporting is based on an activist network in Syria.

It said the Syria Democratic Forces (SDF) alliance, which is conducting the assault to capture the Manbij pocket, had taken 16 villages and was 15 km (9 miles) from Manbij town itself.

The U.S. officials earlier said the operation would be overwhelmingly comprised of Syrian Arabs instead of forces with the Kurdish YPG militia, who will only represent about a fifth or a sixth of the overall force.

This is seen as important to NATO member Turkey, which has opposed any further expansion of Syrian Kurdish sway at the frontier.

Ankara sees the Syrian Kurdish YPG fighters, who already control an uninterrupted 400-km (250-mile) stretch of the border, as terrorists and has been enraged by U.S. backing for the militia in its battle with Islamic State in Syria.

However, the Observatory said the Kurdish YPG militia made up the majority of the fighters taking part in the SDF assault.

SDF and YPG officials could not immediately be reached for comment.

The U.S. officials told Reuters the YPG would only fight to help clear Islamic State from the area around Manbij. Syrian Arab fighters would be the ones to stabilise and secure it once Islamic State is gone, according to the operational plans.

"After they take Manbij, the agreement is the YPG will not be staying ... So you'll have Syrian Arabs occupying traditional Syrian Arab land," the official said.

TURKISH SENSITIVITIES

A U.S. official said Turkey supported the operation but another clarified it was not expected to directly participate militarily.

A Turkish military source said Ankara had been informed by Washington about the operation but could not contribute to it because of the involvement of Kurdish YPG militia fighters and because it was beyond the range of artillery stationed in Turkey.

Still, Turkey has been shelling Islamic State positions in northern Syria by firing across the border in recent weeks.

The operation precedes an eventual push by U.S.-backed Syrian forces toward the city of Raqqa, the Islamic State's de facto capital in Syria and the prime objective in Syria for U.S. military planners.

The U.S. military official said depriving Islamic State of the Manbij pocket would help isolate the militants and further undermine their ability to funnel supplies to Raqqa.

U.S. President Barack Obama has authorised about 300 U.S. special operations forces to operate on the ground from secret locations inside Syria to help coordinate with local forces to battle Islamic State.

One U.S. service member was injured north of Raqqa over the weekend, the Pentagon said.

The YPG has been the most effective ally on the ground for U.S.-led air strikes against IS, and last year captured large areas from it in Hasaka province.

The United States hopes that success will draw more and more recruits from Arab populations in Syria to battle the militants and reclaim territory from it.

U.S. officials caution that territorial gains would not spell the end of Islamic State, which has "metastasized" and established itself outside of its self-declared caliphate in Iraq and Syria, spreading to Libya, Afghanistan and beyond.

"It would be premature to say that the gains in Syria, even if they're sustained, will spell defeat for ISIL, any more than the pummelling of al-Qaeda in Pakistan has meant the end of that group," said one official, using an acronym for the group.

"They aren't nations that will surrender," the official added, saying the ideas driving them were far "harder to kill."

(Additional reporting by Tulay Karadeniz in Ankara and John Walcott in Washington; Editing by Peter Millership and James Dalgleish)

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