RIYADH (Reuters) - The top U.N. humanitarian official in Yemen said on Tuesday he feared for the lives of civilians in two Red Sea coast cities where Yemeni government forces and their Gulf Arab backers are on an offensive to oust Iran-allied Houthi militia.

Coalition-supported Yemeni army forces advanced northward into al-Mokha and Dhubab last week in a bid to deprive the Houthis of strategic Red Sea ports.

“I am extremely concerned about the safety and well-being of civilians in the Dhubab and al-Mokha districts of Taiz Governorate,” U.N. Humanitarian Coordinator in Yemen Jamie McGoldrick said in a statement.

“Repeated air strikes, shelling and sniper attacks in and around the town of al-Mokha have killed and injured scores of civilians and have ground most services to a halt, including the main market and the water supply system.”

Some 20,000 to 30,000 people were trapped in al-Mokha and in need of assistance, he added, but intense fighting was impeding safe access for relief agencies.

Houthi forces attacked a Saudi warship stationed off the coast on Monday night, causing an explosion that killed two crew members and injured three others, according to the Saudi state news agency SPA.

The Houthis said they had launched a ballistic missile at a coalition military base on the Red Sea island of Zuqar between Yemen and Eritrea on Tuesday morning, posting video of the attack on their news channel, al-Masira.