Three U.S. soldiers were killed this week in a rocket attack at a U.S. base near the Iranian border, the military said Thursday, bringing June’s death toll to 15 and marking the bloodiest month for U.S. troops in Iraq in two years.

U.S. military commanders have said in recent months that they feared such an increase in violence would accompany the planned withdrawal of most American troops by the end of the year. Military officials in Baghdad and at the Pentagon blamed the mounting death toll on the growing sophistication of weapons that insurgents and Iranian-backed militia groups are using, including powerful rockets, armor-piercing grenades and jam-resistant roadside bombs suspected of coming from Iran.

The heightened danger underscores the volatile security situation in Iraq, amid ongoing debate here and in Washington about whether any U.S. troops should remain in the country.

June’s death toll was the highest since 15 troops died here in June 2009, according to, a Web site that tracks U.S. military deaths. Fourteen of the deaths were combat-related, the highest since 23 soldiers and Marines were killed in action in June 2008, the site said.

During much of the insurgency that followed the 2003 U.S.-led invasion, U.S. military commanders blamed Sunni-dominated terrorist groups such as al-Qaeda in Iraq for many of the attacks against American troops. But as the U.S. military has adjusted its tactics, largely withdrawing from cities and improving its technological capacity to combat deadly roadside bombs and suicide attacks, officials say it has become far harder for loosely organized Sunni militias to strike out against the roughly 46,000 U.S. troops in the country.

U.S. troop fatalities: June was the deadliest month in two years for U.S. troops in Iraq. (Alicia Parlapiano/The Washington Post)

Now, according to Maj. Gen. Jeffrey S. Buchanan, chief spokesman for the U.S. military in Iraq, the primary threat to the Americans comes from three Shiite militia groups operating in Iraq, which officials said they believe are being trained and equipped by Iranian Revolutionary Guard Corps special forces.

“All of them receive at least indirect support from elements in Iran,” Buchanan said in an interview this week.

In early June, what U.S. officials believe was a sophisticated rocket slammed into a joint Iraqi-U.S. military base in eastern Baghdad, killing six American soldiers in the deadliest single attack on forces here in more than two years. In addition, three U.S. troops were killed by roadside bombs in June.

Last week, an American contractor for the U.S. Agency for International Development was killed when suspected Shiite militants attached a bomb to a car he was riding in near a Baghdad university. And Sunday, two U.S. troops were killed when an apparent armor-piercing grenade was lobbed at their vehicle.

Although the U.S. military did not release specifics on Wednesday’s attack pending notification of next of kin, officials familiar with the incident said the rocket was so powerful that it also wounded more than a dozen soldiers, several critically.

There was no immediate assertion of responsibility for that attack. However, Kataib Hezbollah, one of the Shiite militia groups mentioned by Buchanan, said earlier last month that it was responsible for the attack that killed six soldiers.

Buchanan said there is “no doubt” that Kataib Hezbollah “follows orders” from the Iranian Revolutionary Guard Corps’s Quds Force, a highly specialized unit responsible for operations outside Iran. “Their leadership lives in Iran, they are directly trained by the Quds Force and they are supplied by them,” Buchanan said.

In an interview with Bloomberg News, outgoing Defense Secretary Robert M. Gates expressed similar frustration with Iran’s ties to the Shiite militias operating in Iraq. Iran is “facilitating weapons, they’re facilitating training, there’s new technology that they are providing,” Gates said. “They’re stepping this up, and it’s a concern.”

Buchanan said efforts to protect U.S. forces in Iraq are further complicated by rival Shiite militias that are vying to emerge as the dominant Shiite insurgency group in Iraq.