US forces stepped up security measures at bases across Iraq on Thursday after concluding that a suicide bomber, probably in Iraqi military uniform, was behind Tuesday's blast at a military base in Mosul.

"What we think is likely, but certainly not certain, is that an individual in an Iraqi military uniform, possibly with a vest-worn explosive device, was inside the facility and detonated the facility, causing this tragedy," Brigadier General Carter Ham, the US commander in Mosul, told CNN in an exclusive interview.

Hours after the lunch-time explosion, which killed at least 22 people and wounded 69 others, US military officials speculated that the blast was caused by a rocket.

Ham said investigation was going on over how the suicide bomber had managed to breach the security in the base. "Clearly in this instance I failed to identify some shortcoming that allowed this to occur," he said.

Ham added that the attack was probably "a well-coordinated action" rather than the actions of an individual alone.

The commander noted that he was concerned about "copycat attacks," saying, "We have to be on our best guard over the coming days, weeks and months for that kind of threat."

The admission by the top US general gave credibility to a claim made by the Ansar al-Sunna group that it had sent a "martyr" into the camp and the group vowed to carry out further attacks.

A statement the group put on its Web site said on Thursday that the Americans were stupid for not realizing sooner that it was a suicide attack.

"First they said it was a mortar or rocket, then they said it was a suicide operation with local materials ..." the group said. "Are they really this stupid that they still don't know how they've been hit, or was it too painful to admit?"

The US military said it will launch a strict investigation not only into base security but into the procedures that allow thousands of Iraqis -- police, National Guards and civilians -- into American camps throughout the country.

Source: Xinhua