US forces have fired mortars, helicopter guns and tank rounds in a massive demonstration of firepower around Tikrit, the birthplace of Saddam Hussein. The coordinated night-time operation early on Monday targeted areas from which US forces have been attacked in recent days. But it also was aimed as a show of force meant for residents of the flashpoint area 180km north of Baghdad. The earth shook repeatedly and flashes lit the night skies as ammunition was fired from Abrams tanks, mortar launchers and helicopter gunships. "It is more harassment than interdiction," said Lieutenant-Colonel Steven Russell, who commands the 4th Infantry Division's 1-22 battalion. He stressed it was also more than just a show of force. **'Not mere display'*** "It is not a display as much as going after enemy operations as they occur." The 1-22 battalion had 150 men involved in the operation, with hundreds more from other battalions also participating. The operation underlined the increasingly aggressive stance which the army had adopted since a Blackhawk helicopter was downed just outside the gates of its Tikrit headquarters on 7 November, killing all six aboard. "This fits into a bigger operation where each of the task forces goes after the enemy," said Russell, directing the firepower from a location outside Tikrit. "We will use all the means necessary to maintain security."