Stuart W. Bowen Jr. said in his quarterly report that June was the deadliest month for the U.S. military in more than two years, with 14 soldiers killed, The Washington Post said. Most of the deaths were the work of Shiite militias, he added.
"Iraq remains an extraordinarily dangerous place to work," Bowen wrote. "It is less safe, in my judgment, than 12 months ago."
About 46,000 members of the U.S. military are deployed in Iraq. Most are scheduled to leave in the fall, and U.S. officials are waiting for a decision from Prime Minister Nouri al-Maliki and the Iraqi government on extending a training agreement.
Both Shiite militias and al-Qaida are contributing to the violence, Bowen said. Attacks, including several suicide bombings intended to inflict mass casualties, have killed at least 193 Iraqi police and soldiers and 248 civilians.