SINGAPORE (AFP) - Islamic State (ISIS) extremists in the strategic northern Syria town of Manbij "aspire" to directing international terror plots, US Defence Secretary Ashton Carter said on Friday (June 3).

Anti-ISIS fighters known as the Syrian Democratic Forces, a US-backed alliance of Kurds and Arabs, this week launched an offensive to capture Manbij.

The town is on the north-south axis between Jarablus, on the border with Turkey and controlled by IS fighters, and Raqa, ISIS's self-proclaimed capital in Syria.

Its capture has long been seen as key in stemming the flow of foreign fighters into and out of Syria.

"There are people there, and I can't go into any details, who aspire to inspiring or even directing plots outside of Syria," Carter said at a news conference in Singapore, where he is attending the Shangri-La Dialogue, a high-level security summit.

"That, along with that fact that it is a transit hub for foreign fighter flows in both directions, is a reason why it is an important objective and we are pleased to work with local forces ... who basically want to take back their own territory from ISIL ... and they are making progress," he added, using another acronym for ISIS.

Carter is in Singapore until Sunday for the Shangri-La Dialogue, a meeting of Asian security chiefs to discuss regional security.