Col Steve Warren told reporters that IS was "on the defensive", and had "not gained one inch in Iraq since May".
It had also been driven out of 20% of its territory in Syria, he added.
Despite the losses, IS has continued to launch counter-offensives - including several near the western Iraqi city of Haditha in the past 48 hours.
Col Warren said coalition air strikes had helped Iraqi government forces repel an assault on Monday by about 200 militants, and that more than 100 had been killed.
He did not give a figure for casualties on the government side, but a Sunni tribal commander told AFP news agency that they had lost more than 25 fighters.
Haditha Mayor Mabrouk Hamid said the IS counter-offensive had involved more than 40 armoured vehicles, some of them filled with explosives.
Col Warren said IS had shifted its focus to Haditha, situated near a key dam in the north of Anbar province, after losing control of the provincial capital Ramadi the government last week.
The coalition spokesman also denied claims by IS that it had captured the towns of Barwana and Sakran, near Haditha. He insisted it had not gained any territory in Iraq since May, when Ramadi was overrun in an embarrassing defeat for the army.
In June 2014, IS seized large parts of northern and western Iraq, and proclaimed the creation of a caliphate stretching across the border with Syria.
Iraqi government and Kurdish Peshmerga forces - supported by Iranian-backed Shia militiamen, Sunni tribesmen and coalition air strikes - have since regained more than 20,000 sq km (about 8.000 sq miles), according to the coalition.
IS militants have also been driven out of the city of Tikrit in the past year, but they continue to control Mosul, the largest city in the north.
In Syria, the jihadists have been losing ground to President Bashar al-Assad's forces, rebel groups, and Kurdish militia fighters. But they have also been able to capture new territory of strategic value, including the ancient city of Palmyra.
Islamic State (IS) has lost 40% of the territory it once controlled in Iraq, a spokesman for the US-led coalition battling the jihadist group says.