A rocket hit a base in northern Iraq that hosts U.S. forces on Thursday but caused no casualties, Iraqi security sources said.

The base, known as K1 near the city of Kirkuk, is where a U.S. civilian contractor was killed in December in a similar attack, blamed by Washington on Iran-backed militias.

A Reuters witness said U.S. warplanes had scrambled and were soaring overhead shortly after the attack on Thursday. There was no immediate claim of responsibility for the rocket.

The December attack was followed by retaliatory U.S. strikes that killed some two-dozen militiamen, attacks by crowds on the U.S. embassy in Baghdad and then the killing by U.S. drone strike in Baghdad of top Iranian commander Qassem Soleimani - a series of events that brought the region close to full-scale conflict.

Followers of Soleimani and Iraqi paramilitary leader Abu Mahdi al-Muhandis, who died in the same strike, this week marked 40 days since the deaths of their leaders with ceremonies in Baghdad.

Soure: Reuters