Twitter is working with US law enforcement officials on unspecified threats amid reports that the social network had been targeted for blocking accounts linked to the Islamic State.

"Our security team is investigating the veracity of these threats with relevant law enforcement officials," a Twitter spokesman said, without elaborating.

The news comes following reports that Twitter had suspended accounts linked to ISIS and related entities, including the Nigerian Islamic group Boko Haram.

A page on the online bulletin board Pastebin, the source of which could not be verified, showed an image of Twitter founder Jack Dorsey in what appear to be the crosshairs of a rifle scope and a message in Arabic.

Like other social media operators, Twitter has struggled with allowing free speech while avoiding being used as a tool for violence. Twitter's terms of service bar direct, specific threats against others.

But analysts note that terrorist organisations frequently use Twitter and other social media for recruiting, fundraising and communications.