Updated at 3:20 p.m. ET

A purported affiliate of the self-declared Islamic State has claimed responsibility for a deadly car bomb attack that seriously damaged the Italian Consulate in Cairo, killing one Egyptian.

A group calling itself Islamic State in Egypt posted the claim of responsibility to social media, the authenticity of which could not be immediately confirmed. The ISIS affiliate in Egypt had previously called itself Sinai Province, and it wasn't immediately clear if the claim was from a new branch of the extremist network.

NPR's Leila Fadel reports from Cairo that the attack is the first on a diplomatic mission in Egypt since the state began battling an insurgency nearly two years ago and it is a serious escalation. "It will surely set off alarm bells at other missions in Egypt," she says.

Al-Jazeera English reports that the blast, at one of the busiest intersections in Cairo, ruptured underground water pipes.

Italian Foreign Minister Paolo Gentiloni posted a message on Twitter after the bombing vowing that "Italy will not be intimidated."

Egypt's Prime Minister Ibrahim Mehleb was quoted by a state-run newspaper as saying that his country is "at war" with ISIS.

"The time has come for the world to unite in facing terrorism, which now threatens the entire world," Al-Ahram newspaper quoted him as telling reporters.

The Associated Press reports:

"The bombing struck at around 6:30 a.m. (12:30 a.m. EDT), exploding in a side street in downtown Cairo near the building's back entrance and a busy highway overpass. Italian authorities said the consulate was closed at the time and none of its workers were wounded.

"The blast killed a passer-by and wounded eight, one of whom is still in the hospital, the Interior Ministry said in a statement. Egyptian Heath Ministry official Hossam Abdel-Ghaffar told the AP that the man killed had some of his limbs blown off."

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