Iraqi forces recaptured territory from advancing Islamic State militants near the recently-fallen city of Ramadi on Sunday, May 24, while in Syria the government said the Islamists had killed hundreds of people since capturing the town of Palmyra, according to Reuters.

The fall of Ramadi and Palmyra, on opposite ends of the vast territory controlled by Islamic State fighters, were the militant group's biggest successes since a U.S.-led coalition launched an air war to stop them last year.

In a sharp criticism, U.S. Defense Secretary Ashton Carter accused Iraq's army of abandoning Ramadi, a provincial capital west of Baghdad, to a much smaller enemy force.

"The Iraqi forces just showed no will to fight," he told CNN's State of the Union program. "They vastly outnumbered the opposing force, and yet they withdrew from the site."

Iraq's government, along with Iran-backed Shi'ite militiamen and locally-recruited Sunni tribal fighters, launched a counter-offensive on Saturday, a week after losing Ramadi. A police major and a pro-government Sunni tribal fighter in the area said they had retaken the town of Husaiba al-Sharqiya, about 10 km (6 miles) east of Ramadi.

Days after taking Ramadi, Islamic State also defeated forces of the Syrian government of President Bashar al-Assad to capture Palmyra, home to 50,000 people and site of some of the world's most extensive and best-preserved Roman ruins.

The fighters have killed at least 400 people, including women and children in Palmyra since capturing the ancient Syrian city four days ago, Syrian state media said on Sunday.

Reuters says it was not immediately possible to verify that account, but it was consistent with reports by activists that the Islamist fighters had carried out executions, leaving hundreds of bodies in the streets.

Islamic State supporters have posted videos on the Internet they say show fighters going room to room in Palmyra's government buildings, searching for hiding troops and pulling down pictures of Assad and his father.

The Syrian Observatory for Human Rights, which monitors violence in the country with a network of sources on the ground, says beheadings have taken place in the town since it fell but has not given an estimate for the toll among civilians. It says at least 300 soldiers were killed in the days of fighting before the city was captured.