The United States is not at war with Yemen's Iran-backed Houthis, the Pentagon said on Thursday, even as it detailed its fifth round of strikes against the group in the past week.

Pentagon spokesperson Sabrina Singh said the strikes, including Thursday's targeting of anti-ship missiles, were purely defensive in nature.

"We are not at war with the Houthis," said Singh.

"The Houthis are the ones that continue to launch cruise missiles, anti-ship missiles at innocent mariners. ... What we are doing, with our partners, is self-defense," she said.