As US-led Arab and Kurdish forces were fighting in the last IS territorial holdout in Syria, 13 defence ministers of the anti-IS coalition met on the sidelines of the Munich Security Conference.

Shanahan, the US acting defence secretary, pledged ongoing backing for the fight -- but kept allies guessing as to how that would be achieved once US forces pull out, and won no solid pledges of support.

"While the time for US troops on the ground in northeast Syria winds down, the United States remains committed to our coalition's cause: the permanent defeat of ISIS, both in the Middle East and beyond," he said.

He pledged that the US would "maintain our counterterrorism capabilities in the region" and "continue to support our local partners' ability to stand up to the remnants of ISIS" -- but gave no details about how this would be done.

- 'In together, out together' -

IS fighters have been boxed into an area of around one square kilometre (less than half a square mile) by a US-backed Arab-Kurdish militia in a last battle over the remaining patch of territory the militants control in northeastern Syria.

Once they are defeated, US troops are set to withdraw from Kurdish-controlled areas after President Donald Trump in December announced the pullout of around 2,000 US troops.

The decision stunned allies including France, which contributes artillery and about 1,200 forces in the region, including soldiers who train Iraqi troops.

"It is totally out of the question to have French troops on the ground without the Americans there," one French government source told AFP. "It's just no."

German Defence Minister Ursula von der Leyen, whose country has helped with surveillance flights and logistical support, stressed that the idea of the mission should be "in together, out together".

"Joint decisions should be the rule for Syria and Iraq where almost all NATO members took part in the fight against the IS," she said.

A senior US defence official said that none of the allies in the IS coalition had made any "specific commitment... either whether they would stay or (whether) they would leave when we have left".

There was "a tremendous desire to have a security arrangement or mechanism," the official said, but conceded that no concrete solution had been found to "resolve the security vacuum".

- 'Going underground' -

The imminent collapse of the IS "caliphate" in Syria has increased concerns about experienced militants and foreign fighters escaping and forming new cells in Syria or beyond.

Von der Leyen stressed that "the IS is now changing its face and is going underground and building networks, including with other terror groups and including global networks".

Shanahan said the anti-IS coalition was evolving "to meet the global threat posed by ISIS's offshoots and its murderous ideology" as far away as Afghanistan and the Philippines.

However, the key concern of US allies now is Syria, where the US pullout will leave a vacuum and where major powers -- crucially Russia, Iran and Turkey -- are jostling for influence.

Russian President Vladimir Putin, a leading supporter of the Damascus regime, has called the expected US withdrawal "a positive step that would help stabilise the situation in this region, where ultimately the legitimate government should re-establish control".

Once US forces leave, another complication emerges: the future of areas in northern Syria controlled by the Kurdish YPG militia, a key US ally in the fight against jihadists but branded terrorists by Turkey.

Questions remain over whether Ankara will follow up on President Recep Tayyip Erdogan's warnings that Turkish forces may launch operations against the Kurds to curb their influence along the frontier.

- Observation force -

Istanbul and Washington have called for the creation of a "security zone" to separate YPG-controlled areas from the Turkish border as the US and Turkey increasingly align their positions.

Washington's suggestion of installing an observation force in a buffer zone in Syria's north has the twin objectives of avoiding a Turkish assault on Kurdish forces and halting any jihadist resurgence.

British defence minister Gavin Williamson at a NATO meeting this week did not rule out a UK role, saying that "we will continue to do all that is required to ensure that Britain and our allies remain safe".

However, one high-ranking European military official pointed to the massive challenge of creating such a security zone.

"Securing a buffer zone of an estimated 400 kilometres (250 miles) in length and 30km in width would require around 20,000 troops," said the official.