The Abbott government has expressed ''deep concern'' and experts have warned of a lengthy sectarian conflict across the Middle East after fighters linked to al-Qaeda seized the city of Fallujah.

Under the banner of the Islamic State of Iraq and Syria (ISIS), formerly known as al-Qaeda in Iraq, the militants have reportedly seized control of parts of Anbar province, west of Baghdad, creating a cross-border menace with allied extremists in civil war-torn Syria.

Two years after the last US troops pulled out of Iraq, violence has been steadily worsening, amid sectarian clashes between Sunnis and Shiites of the kind that have engulfed Syria and Lebanon.

Foreign Affairs Minister Julie Bishop said the reports were particularly troubling after extremist violence elsewhere in the region. ''This is a matter of deep concern, particularly due to ongoing terrorist activities in Syria and neighbouring countries. The Australian government is monitoring the situation and is concerned by reports of dual nationals from Australia becoming involved in the Syrian conflict. It is not only a highly dangerous situation but is also against Australian law.''