Last Updated Mar 5, 2015 7:51 AM EST
KIRKUK, Iraq -- The front line in Iraq's Kirkuk province was as close as CBS News correspondent Holly Williams and her team could get Thursday to the battle for Tikrit.
The black flags of the Islamic State of Iraq and Syria (ISIS) flew just 30 yards from Williams' position with Kurdish peshmerga fighters, and beyond that, in ISIS territory, was Iraq's main north-south highway.
Williams reported that Iraqi forces need to recapture the vital road if they're going to defeat the extremists and reclaim Tikrit.
But a new ISIS video released Thursday claims to show the militants fighting off the assault on Tikrit. On the city's outskirts, Iraqi forces were still trying to encircle the extremists and cut off their supply routes.
They have been set back by roadside bombs littering the roads into the city, and by ISIS suicide bombers, since the offensive began on Monday.
Saddam Hussein's hometown was a pocket of fierce resistance after the U.S. invasion of Iraq, but since last June, Tikrit has been an ISIS stronghold.
Fighting alongside the Iraqi soldiers and peshmerga are Iranian troops -- and Iranian generals are reportedly commanding at least part of the battle.
"They realize their close allies are threatened and I think they would rather deal with ISIS here than they would on the gates of Iran," said U.S. Army Col. Harry Schute (Ret), who took part in the 2003 U.S. invasion.
Schute told Williams the battle for Tikrit could prove decisive in turning the tide against ISIS -- which now controls about a third of Iraqi territory.
"It's a test to see if the Iraqi security forces are up to really going after a major urban area," said Schute, noting the expected battle later this year for the much larger urban area of Mosul.
As Iranian influence in Iraq increases, the U.S. has had no direct involvement in the fight for Tikrit.
American officials say that's because the Iraqi government did not request any airstrikes.