The Islamic State in Iraq and the Levant (ISIL) battled with rival opposition fighters in northern Syria on Sunday, using US-made military vehicles captured from neighbouring Iraq for the first time, a monitoring group said.
ISIL has made rapid gains in Iraq in the past two weeks, taking control of the northern city of Mosul and major border crossings with Syria.
The advances of ISIL in Iraq appear to have spurred on the Syrian branch, which is fighting both the army of President Bashar al-Assad and also rival opposition groups such as the Western-backed Free Syrian Army.
For the first time, ISIL combatants were using US-made Humvees - four-wheel-drive military vehicles - in fighting in northern areas of Syria's Aleppo province, the British-based Syrian Observatory for Human Rights said.
The vehicles, which appear to have been seized during ISIL's recent Iraqi offensive, were used to gain control of villages outside the town of Azaz, close to the Turkish border, it said.
The Observatory, an anti-Assad group that tracks events on the ground through activists, said ISIL in Syria had been supplied with dozens of the vehicles from Iraq.
The United States has long supplied Humvees to the Iraqi army, which has been fighting a violent insurgency since American forces withdrew at the end of 2011.
ISIL fighters in Iraq have often seized abandoned military equipment from Iraqi forces, including armoured vehicles.
The capture of border areas by ISIL on Sunday is likely to make it easier for the group to transfer equipment and fighters between the countries, where the conflicts have fed off each other.
As in Syria, ISIL has started to clash with other Sunni militias in Iraq.