The U.S. military on Tuesday carried out a new strike in Yemen against four Houthi anti-ship ballistic missiles, two U.S. officials told Reuters, the latest move against the Iran-aligned group over its targeting of Red Sea shipping.

One of the officials said the missiles were struck because they were being prepared to target ships in the region. The U.S. strike came a day after Houthi forces hit the U.S.-owned and operated dry bulk ship Gibraltar Eagle with an anti-ship ballistic missile.

Houthi attacks on Red Sea shipping have continued even after the U.S. and Britain last week launched an initial wave of strikes to degrade Houthi capabilities.

But unlike the initial U.S. strikes last week, which were against pre-planned targets, Tuesday’s strikes appeared to show that the U.S. military would proactively go after Houthi military capabilities as they are detected. If confirmed, that would usher in a far more assertive posture for the U.S. military toward the Houthis.

The U.S. military’s Central Command on Monday disclosed the first seizure in more than four years of advanced Iranian-manufactured ballistic missile and cruise missile components, in a Jan. 11 operation that saw two U.S. Navy SEALs lost at sea near the coast of Somali.

“Initial analysis indicates these same weapons have been employed by the Houthis to threaten and attack innocent mariners on international merchant ships transiting in the Red Sea,” Central Command said in a statement.