The Nigerian terrorist organization Boko Haram has pledged allegiance to the Islamic State (ISIS) terror group.

“We announce our allegiance to the Caliph of the Muslims… and will hear and obey in times of difficulty and prosperity, in hardship and ease, and to endure being discriminated against, and not to dispute about rule with those in power, except in case of evident infidelity regarding that which there is a proof from Allah,” a man claiming to be Boko Haram leader Abubakar Shekau said on Twitter, the Associated Press reported.

While the level of cooperation between the two terrorist groups is unclear, the move by Boko Haram makes it the largest terror group to pledge allegiance to Islamic State.

According to the New York Times, Boko Haram is estimated to have up to 6,000 fighters and controls an area of about 8,000 square miles in northeastern Nigeria.

Meanwhile, Islamic State jihadists have destroyed the ruins at the ancient city of Hatra, Iraqi officials say.

Located about 68 miles from Mosul, Hatra was built by the Seleucid Empire in the 3rd or 2nd century BCE, and later became an important frontier city for the Parthian Empire, withstanding several attacks by the Roman Empire.

A UNESCO world heritage site, Hatra was famed for its fusion of Greek, Mesopotamian, Caanite, Aramean, and Arabian temples.

“The destruction of Hatra marks a turning point in the appalling strategy of cultural cleansing under way in Iraq,” UNESCO director-general Irina Bokova said in a statement. “With this latest act of barbarism against Hatra, [Islamic State] shows the contempt in which it holds the history and heritage of Arab people.”