New York City police are assessing protection plans for polarising anti-Islam blogger Pamela Geller after what authorities called a "direct threat" from Isis to "slaughter" her.

In a posting believed to be from Isis on the anonymous message board JustPasteIt, the group warned it was sending "all our Lions to achieve her slaughter."

The message adds, apparently referring to the failed attack by two gunmen on the "Draw Mohammed" event: "Our aim was the khanzeer (swine) Pamela Geller and to show her that we don't care what land she hides in or what sky shields her."

Geller shot back in an email to the New York Daily News: "This threat illustrates the savagery and barbarism of the Islamic State. They want me dead for violating Sharia blasphemy laws. What remains to be seen is whether the free world will finally wake up and stand for the freedom of speech, or instead kowtow to this evil and continue to denounce me."

Geller, 55, lives on Manhattan's Upper East Side neighbourhood. "We have been in touch with Pamela Geller," police spokesman Stephen Davis told the News. "People known to be associated with Isis have posted a direct threat to her. To whatever extent she will be in New York City, we will do a comprehensive threat assessment to determine what, if any, measures, to take."

Geller, president of The American Freedom Defence Initiative, organised last week's "Draw Mohammed" event in Garland, Texas, where two gun-toting Isis sympathizers were shot dead. After the shootings, Geller posted on her blog: "This is a war. This is war on free speech. Are we going to surrender to these monsters?"

She has a history of controversy for various anti-Islam actions. The non-profit Southern Poverty Law Center has categorised Geller's organisation, Stop the Islamisation of America, as a hate group, labeling it "anti-Muslim." She went to court in 2012 to protect her free speech right to post provocative ads in the New York subway implying Muslims — or, possibly, anyone opposed to Israel — are savages. In any "war between the civilised man and the savage, support the civilised man. Support Israel. Defeat Jihad," the ads stated.

Her latest bid to post ads saying "Killing Jews is worship that draws us close to Allah" on city buses and trains triggered the Metropolitan Transportation Authority's decision last month to ban all political advertising as a legal strategy to avoid Geller's messages.

One Geller critic insisted to the News that the threat from Isis is the kind of response she had hoped to provoke. "She is going to be milking this for all it's worth," he said.