Jihadists with the Islamic State of Iraq and Syria (ISIS), the al Qaeda offshoot that made gains in seizing control of much of northern and north-central Iraq, is making a third attempt Friday to seize Samarra, a city home to a Shiite shrine that spurred sectarian violence when it was bombed in 2006.

Witnesses in Dur, which is between ISIS-held Tikrit and Samarra, told Agence France-Presse that they saw “countless” ISIS vehicles carrying militants.

Other witnesses in Samarra, about 70 miles north of Baghdad, said they saw gunmen gathering on the outskirts of the city to the north, east and southeast. A Samarra tribal leader told AFP that ISIS fighters urged Iraqi security forces at the Al-Askari shrine to leave peacefully.

ISIS also asked tribal leaders to gather forces to protect the shrine and residents of Samarra, but security forces refused to withdraw from the area.

ISIS, which gained control of Iraq’s second-largest city, Mosul, earlier this week, made two previous attempts to seize Samarra. Their efforts were rebuffed by Iraqi security forces in the two attacks on Wednesday and another late last week.

Militants bombed the shrine in February 2006, which led to the deaths of tens of thousands of people as sectarian violence erupted between Iraq’s Shiite majority and Sunni Arab minority.