The United States has reinstated sanctions on Yemen’s Houthi rebels effective Friday, following their continued attacks in the Red Sea and Gulf of Aden, endangering maritime security.

The Houthis were first designated as a terror group three years ago and subsequently delisted due to humanitarian concerns. The relisting follows repeated demands from the U.S. and other countries for the Houthis to stop firing on commercial shipping.

Those demands have been ignored, and attacks have continued despite a series of airstrikes by the U.S. and Britain aimed at taking out radar systems and launch sites used in the attacks.

Earlier on Friday, a missile was launched from Yemen, hitting the port side of the India-bound Panamanian-flagged M/T Pollux, which was transporting crude oil. The extent of the damage is presently unclear, but the M/T Pollux is continuing its journey south under its own power.

Houthi leaders have declared that the group will persist in its attacks in solidarity with the Palestinians, as long as Israel continues what the group termed its crimes against them.

A spokesperson from the U.S. State Department noted that on January 17, Washington announced its intention to relist the Houthis as a Specially Designated Global Terrorist, or SDGT, after 30 days, giving the Iran-backed rebels “the opportunity to scale down their attacks” and “to minimize de-risking across the industry.”

The spokesperson also accused Iran of aiding the Houthis in destabilizing the region.

“Iran has been deeply involved in planning the operations against commercial vessels in the Red Sea. This is consistent with Iran’s long-term materiel support and encouragement of the Houthis’ destabilizing actions in the region. Houthi forces have employed various Iranian-origin missiles and uncrewed aerial vehicles against military and civilian targets throughout the region,” the spokesperson said.

U.S. officials said they have made concerted efforts to mitigate the impact of this designation on the Yemeni people. Washington has actively engaged the shipping industry, financial institutions, banks and humanitarian aid organizations to ensure comprehensive understanding of the broad exemptions associated with this designation.

In the waning hours of former U.S. President Donald Trump’s administration in January 2021, the Houthis were designated as both an SDGT and a foreign terrorist organization, or FTO.

In February 2021, U.S. Secretary of State Antony Blinken delisted the Houthis as both a foreign terrorist organization and as specially designated global terrorists.

This action was taken as the Biden administration aimed to facilitate a diplomatic resolution to the long-running civil war between the internationally recognized government of Yemen, based in the southern port city of Aden, and the Houthis, whose capital is Sanaa.

Additionally, the delisting aimed to make it easier to deliver food and humanitarian aid to the people of Yemen.

The two designations carry distinct penalties. Being named as a specially designated global terrorist empowers the U.S. Treasury Department to disrupt terrorists’ access to funds within the United States and across the international financial system.

On the other hand, designation as a foreign terrorist organization by the State Department prohibits anyone from providing the group with “material support,” including fighting for the group, or providing financial assistance or training.

Members of foreign terrorist organizations who are not U.S. citizens are typically banned from entering the United States, except where there is a rare and high-level decision otherwise. The Houthis have not been relisted as an FTO at this time.

U.S. defense officials said the Houthis have launched dozens of attacks on commercial vessels and naval vessels since mid-November, impacting citizens, cargo and vessels from more than 50 countries.