Yemen’s Houthi rebels announced Tuesday they will resume attacks on Israeli-linked ships in the Red Sea, the Arabian Sea, the Bab al-Mandab Strait, and the Gulf of Aden, ending a period of relative calm that began in January with the cease-fire in Gaza.

“This ban will remain in effect until the crossings to the Gaza Strip are reopened and humanitarian aid, including food and medical supplies, is allowed to enter,” the group said in a statement emailed Wednesday, adding that the decision takes immediate effect.

The announcement follows the expiration of the Houthis’ ultimatum regarding aid deliveries to Gaza. Last Friday, rebel leader Abdul-Malik al-Houthi warned the group would resume naval operations if Israel did not lift restrictions on humanitarian aid within four days.

Since the Gaza war began, the Houthis have fired dozens of missiles and drones toward Israel and targeted ships they believe are linked to it, claiming to act in solidarity with Palestinians. During this period, the rebels have sunk two ships, seized another, and killed at least four sailors in an offensive that disrupted global shipping and forced companies to reroute vessels around southern Africa.

The Houthis had halted attacks after a fragile Gaza cease-fire took effect Jan. 19.

On March 2, Israel blocked the entry of humanitarian aid trucks into Gaza as negotiations with Hamas stalled, prompting intervention from Egyptian and Qatari mediators.

In early March, the U.S. State Department designated the Houthis as a “foreign terrorist organization,” following a directive from President Donald Trump.