Online backers of the Sunni Islamist militants who seized swaths of Iraq this month have said that any US air strikes on the fighters will lead to attacks on Americans.

US President Barack Obama has offered up to 300 American advisers to Iraq to help halt the advance by militants from al Qaeda offshoot the Islamic State in Iraq and the Levant (ISIL).

Washington has so far held off granting a request by Prime Minister Nuri al-Maliki's Shi'ite-led government for air strikes.

A Twitter account with 21,000 followers naming itself the "League of Supporters" called for ISIL sympathisers to post messages online on Friday warning the US not to carry out any strikes.

"This campaign reflects the messages sent by all the Sunni people all over the world to the American people ... (It's) a threat to every American in the event of an American strike on Iraq," the message read.

Among hundreds of supportive responses, one user posted, "As our martyred sheikh Osama bin Laden said, you need not consult anyone about killing Americans."

"American intervention in the affairs of the Islamic State in Iraq and the Levant means that the American infidel is a target for the strikes of the holy warriors anywhere," wrote another.

While lacking an official presence in social media, ISIL has a broad following online. The group regularly circulates high-quality film footage of its exploits in battles across Syria and Iraq.-Reuters