TEHRAN, Young Journalists Club (YJC) - State news agency TAP said nine had died, while the interior ministry put the initial death toll at six. Officials could not immediately be reached for comment, but both tolls were the highest in any attack since 2015.
The police unit from Gar Dimaou in the region of Jendouba was ambushed during a regular patrol, TAP reported.
"The terrorist attackers threw a grenade at the first security car and there were confrontations with firearms," its report cited a security source as saying.
Tunisia became a target for militants after being hailed as a beacon of democratic change with an uprising against autocrat Zine Abidine Ben Ali in 2011.
Some militants operate in remote areas near the border with Algeria.
Tunisia suffered three major attacks in 2015, including two against tourists, one at a museum in Tunis and the second on a beach in Sousse. The third targeted presidential guards in the capital, killing 12. All three attacks were claimed by Daesh.
Tourism, after collapsing, has since gradually recovered, Reuters reported.
The government has maintained a state of emergency, allowing it greater powers in its attempts to dismantle militant networks.