ISIS claimed responsibility for the attack in central Samarra.
Terrorist gunmen wearing suicide vests stormed a police station in the Iraqi city of Samarra Monday, sparking clashes with the security forces, officials said.
"There was a terror attack on Mutawakil police station, now the Iraqi forces are besieging them," interior ministry spokesman Saad Maan told reporters.
The ISIS claimed responsibility for the attack in central Samarra, a city 110 kilometres (70 miles) north of Baghdad, via its propaganda agency Amaq.
Maan said the attackers holed up inside the police station were exchanging fire with the security forces besieging them.
A police major from Salaheddin province where Samarra is located said five attackers had already been killed and added that reinforcements had been deployed.
Samarra is home to a major Iraqi security headquarters and to an important Shiite shrine where a 2006 bombing touched off two years of sectarian bloodletting.
ISIS in Iraq is mostly focused on defending its last major urban stronghold of Mosul, but has launched a number of diversionary attacks elsewhere in the country since the start on October 17 of a broad offensive by the security forces to retake the northern city.
ISIS claimed a similar attack in Samarra on November 28 in which five gunmen wearing suicide vests killed at least four member of the security forces after infiltrating the city.
Iraqi security officials did not immediately release information on any casualties of Monday's attack among police ranks.