The United States on Wednesday imposed sanctions on three men it said funneled money from Kuwait to Islamic militant groups in Iraq and Syria.

The U.S. Treasury Department said two of the men raised and delivered money for the Nusra Front, a Syrian rebel group linked to al Qaeda. A third man helped to transfer money from Kuwait to Syria for the Islamic State in Iraq and the Levant (ISIL), and also helped smuggle fighters to join insurgents in Afghanistan and Iraq.

ISIL, now known simply as the Islamic State, has taken over large swathes of Iraq in the past two months and seeks to impose a caliphate to rule over all Muslims.

"We and our international partners, including the Kuwaiti government, need to act more urgently and effectively to disrupt these terrorist financing efforts," David Cohen, Treasury's undersecretary for terrorism and financial intelligence, said in a statement.

Kuwait has been one of the biggest humanitarian donors to Syrian refugees through the United Nations, but it has also struggled to control unofficial fundraising for opposition groups in Syria by private individuals.

Treasury's sanctions freeze any assets the three men may hold in the United States and prohibit American people and firms from dealing with them.

Unlike some other Gulf states, U.S. ally Kuwait is against arming rebels fighting to topple Syrian President Bashar al-Assad. But it has tolerated fundraising in private houses, mosques and on social media.

Earlier this year, Treasury's Cohen accused Kuwait's former justice minister of promoting terrorism funding and calling for jihad. The minister subsequently quit.

In a speech in Washington in March, Cohen said Kuwait and Qatar in particular needed to do more to prevent humanitarian donations from getting channeled to militant groups.