A U.S. soldier at an Iraqi training base was injured by gunfire directed at the base, marking the first time an American soldier has been wounded by fire from the Islamic State of Iraq and Syria (ISIS), CBS News national security correspondent David Martin reports.

A Pentagon spokesman told Martin that the soldier received superficial wounds to his face after the incident, which occurred Wednesday at 3:00 a.m. Iraqi time.

According to the spokesman, two soldiers were in a guard tower when they heard the crack of gunfire. One of them poked his head above a concrete barrier and caught some of the ricochet in his face. He was treated and returned to duty.

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The spokesman also said the intelligence community estimates that ISIS has lost 25 percent of the ground it once held in Iraq -- between 4,100 and 5,200 square miles. The official's definition of lost ground was territory in which ISIS no longer has freedom of movement and is now in the control of friendly forces.

However, CBS News correspondent Holly Williams reports that ISIS is unquestionably still a threat. On Wednesday, at least ten people were killed in a wave of coordinated car bombs in Ramadi, west of Baghdad.

ISIS fighters are still hanging on in Tikrit, more than ten days into an Iraqi offensive in that city that's being conducted without the support of U.S.-led airstrikes.

Those strikes have weakened ISIS, according to U.S. officials who say 8,000 militants have been killed. CBS News crews have seen the wreckage of armored vehicles and buildings used by ISIS and destroyed by the strikes.

Actually destroying ISIS requires effective ground forces, because you cannot bomb them out of cities without massive civilian casualties, Williams says.

Those forces do not currently exist in Iraq, which is relying on a patched-together army of some government soldiers, local tribesmen and Shiite Muslim militias. So far , those troops are making slow progress.

Most of the ground lost by ISIS has been regained by Kurdish peshmerga forces. The peshmerga are foot soldiers facing off with the extremists on a 600-mile long front line in Sinjar province.

They have the best track record of defeating the militants, but they're massively out-gunned.

Last month, Williams reported that peshmerga forces battling ISIS in northern Iraq say their lives depend on getting military supplies from the U.S.