Syrian Kurdish-led insurgents seized territory on Wednesday from Islamic State in northern Syria as they launched a US-backed offensive against the extremist militia, the second front opened against the jihadist group in a week.

The Democratic Forces of Syria (DFS) began the attack overnight on the Islamic State-controlled city of Minbij, less than 20 kilometres from the Turkish border, taking control over 20 villages and farms, according to the Syrian Observatory for Human Rights.

The US-led coalition against Islamic State said it had carried out 18 airstrikes in the area the previous day. The Observatory said 15 civilians were killed in the strikes and five bridges connecting the Minbij area to the Islamic State-held town of Jarablus further north were destroyed.

The move in Minbij comes after the DSF - composed of several key Arab units, but largely dominated by the Kurdish People's Protection Units (YPG) - launched an attack last week on areas north of al-Raqqa, the de facto Islamic State capital in Syria.

Taking Minbij, in Aleppo province, is seen as key to putting pressure on Islamic State in al-Raqqa, by cutting the city off from other areas held by the extremists and their supply routes, including the Turkish border area.

"The operation is aimed at isolating Islamic State in northern Syria," Idriss Nassan, a senior Kurdish official, told dpa. 

DSF spokesman Brigadier Talal Silo told dpa that the Minbij operation is being led by Arab forces from the so-called Minbij Military Council, which is linked to the opposition's Free Syrian Army.

The Kurds are supplying logistical backing and support, he said.

Turkish Foreign Ministry officials declined to comment on the offensive. Turkey has long voiced opposition to Kurdish forces advancing in the so-called Minbij pocket, west of the Euphrates River, in order to take on Islamic State.

"No one can pass to the west of the Euphrates, we are very determined about this," Turkish President Recep Tayyip Erdogan said last year.

Kurdish forces have been present just west of the river for months, but, largely because of Turkish objections, they have refrained from advances, according to Kurdish officials.

Ankara is wary of the Kurdish forces because of their links with the Kurdistan Workers' Party (PKK), which has been fighting against the state for more than 30 years for greater rights for the Kurdish minority.

Washington makes a distinction between the PKK and YPG, and US officials have stressed that they do not want to have to choose between Turkey and the Kurds. The YPG has been the most effective US ally in Syria in pushing back Islamic State and seizing land from extremists.

The Kurds meanwhile set down a marker for a further possible expansion, with the Hawar news agency, which is close to the dominant Kurdish Democratic Union Party (PYD), reporting the formation of a local council for the city of al-Bab.

Al-Bab, south-west of Minbij, is the other main city in the Minbij pocket. Hawar announced the formation of a similar council for Minbij in April.

Islamic State has been ceding territory in Syria and Iraq. However, the extremists made advances last month against Syrian rebels, including groups backed by Turkey, in north-western Aleppo, threatening a key border crossing.

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