It followed another attack over the weekend in which rockets were fired from northern Iraq at a base in Syria that houses forces from the US-led coalition against the Islamic State jihadist group.

US troops in the Middle East have been attacked scores of times since mid-October, but the cycle of rocket and drone attacks and periodic retaliatory strikes stopped following heavy American air raids on Iran-linked targets in early February.

"Yesterday there was an attack against Al-Asad Air Base" that did not cause injuries or damage, the defense official said on condition of anonymity, without specifying the nature of the attack.

"This was the second attack against US forces since February 4," the official said.

A wave of more than 165 attacks on US troops in Iraq and Syria began in October shortly after the start of the latest round of conflict between Israel and militant group Hamas, causing injuries but no deaths among US forces.

But a drone attack in Jordan killed three US soldiers in late January, after which Washington carried out strikes on dozens of Iran-linked targets in Iraq and Syria. The air raids were followed by a pause in the attacks that lasted until late this month.

The majority of the attacks were claimed by the Islamic Resistance in Iraq, a loose alliance of Iran-backed groups angered by US support for Israel, but there have been no claims for either of the attacks this month.