Coalition strikes 'pound Islamic State stronghold' in northern Syria

The US-led international coalition fighting the Islamic State group said it had conducted 18 airstrikes near the strategic town of Manbij in northern Syria, opening a new front against the militants.

The announcement of the strikes followed word from the Syrian Observatory for Human Rights that US backed Kurdish and Arab fighters had thrust into the key militant-held pocket along the Turkish border.

The strikes over the past 24 hours destroyed an IS headquarters, communication towers, six IS-used bridges and eight IS fighting positions, among other targets, according to a coalition statement sent out by Centcom.

"The destruction of ISIL targets in Syria and Iraq further limits the group's ability to project terror and conduct operations," said the statement, using an alternate acronym for the IS militants.

IS-held Manbij is located in Aleppo province, some 20 miles (30 kilometres) west of the Euphrates.

The swath of territory controlled by IS on either side of the Euphrates River has long been a key target for Washington as it is seen as the main entry point for foreign fighters.

The Pentagon has deployed more than 200 Special Forces troops alongside the Syrian Democratic Forces (SDF), a Kurdish-led alliance in which it has been trying to boost the Arab element.

SDF military adviser Nasser Haj Mansour told AFP earlier that the alliance's fighters were heading from the Tishreen Dam on the Euphrates towards Manbij.

"The clashes are fierce and intense," he said.

The offensive is one of two the SDF has launched against IS in northern Syria in recent weeks.

Last month, the alliance launched an assault on the extremists north of their de facto Syria capital Raqqa, seizing dozens of villages in the north of Raqqa province.

Washington sees the SDF – which is dominated by the Kurdish People's Protection Units (YPG) – as the most effective ground force against IS in Syria.

With the help of coalition airstrikes, the SDF has cleared IS militants from large parts of Syria's northeast.

But US support for the Kurdish-led alliance has angered NATO ally Turkey.

Ankara regards the YPG as a branch of the rebel Kurdistan Worker's Party (PKK), which has fought a three decade insurgency against the Turkish state.