SINGAPORE - Islamic State fighters have used the northern Syrian city of Manbij as a base to hatch plots against Europe, Turkey and the United States, necessitating a US-backed offensive against it, US Defense Secretary Ash Carter said on Thursday. US officials disclosed to Reuters this week that thousands of US-backed fighters in Syria were launching an offensive to capture a swath of land known as Manbij pocket after weeks of quiet preparations. Carter, who was en route to a regional security meeting in Singapore, told reporters the Arab-led force, which the United States was supporting, was moving towards Manbij city. The city was a route for the flow of foreign fighters, making it “essential and important for the counter-ISIL fight in Syria,” he said. “We know that there is external plotting, from Manbij city ... against the homelands of Europe, Turkey - all good friends and allies of ours - and the United States as well,” Carter said.

“This is capable force. They are doing all the things we can always do with able and motivated local forces to fight ISIL.” He did not elaborate on the plotting he was referring to.

The operation, which began Tuesday and could take weeks to complete, aims to choke off Islamic State’s access to Syrian territory along the Turkish border that militants have long used as a logistics base for moving foreign fighters to and from Europe.

US officials said a small number of US special operations forces would support the offensive on the ground, acting as advisers and staying back from the front lines.

The operation would also count on support from US-led coalition air strikes and ground-based firing positions across the border in Turkey, a US ally in NATO.

It comes ahead of an eventual push by the US-backed Syrian forces toward Raqqa, the Islamic State’s de facto capital in Syria and the prime objective in Syria for US military planners.

The US military official said depriving Islamic State of the Manbij pocket would further isolate them and undermine their ability to funnel supplies to Raqqa.

US President Barack Obama has authorized about 300 US special operations forces on the ground in secret Syrian locations to battle militants in coordination with local forces.

Carter said his Singapore talks would include discussions with regional Muslim countries on cooperation in the fight against Islamic State militants and reconstruction in Iraq.