British lawmakers have opened debate on whether to join the United States and a coalition of Western and Arab nations in airstrikes meant to thwart Islamic State group militants in Iraq.

Lawmakers are expected to approve the motion, which is supported by all three main parties and comes only days after Iraq's prime minister requested help. The motion does not address any action in Syria. Critics say that would be illegal because Syrian President Bashar Assad has not invited outsiders to help.

Foreign Secretary Philip Hammond refused to speculate Friday on how long the military campaign could last, but lawmakers envision a long-term action.

"We are going into this with our eyes open," Hammond told Sky News, that adding the Islamic State group is a threat to national security.