The US has opposed a call for a referendum on independence for Iraq’s autonomous Kurdish region.

The call from Massud Barzani, president of the Kurdish region in northern Iraq, came as Sunni militants from the Islamic State of Iraq and Greater Syria (ISIS) are continuing their offensive after seizing a large part of Iraq’s northern and western territories and declaring the establishment of an Islamic caliphate in areas under their control in both Iraq and Syria.

Barzani reportedly told the Kurdish regional parliament Thursday to make preparations “to organise a referendum on the right of self-determination”.

“The fact is that we continue to believe that Iraq is stronger if it’s united,” Xinhua quoted White House spokesman Josh Earnest as telling reporters at a daily news briefing.

“That’s why the United States continues to support an Iraq that is democratic, pluralistic and unified,” he said. “And we’re going to continue to urge all parties in Iraq to continue working together toward that objective.”

Washington, while stepping up military aid to the Iraqi government headed by Nuri al-Maliki, is pressing for an “inclusive ” government in the country as part of efforts to combat the ISIS’s advances.

“The best way for Iraq to confront the threat that’s posed by ISIL (now known as ISIS) is to unify the country in the face of that existential threat,” Earnest said. “And we think that’s in the best interest of all the citizens of Iraq.”