(CNN) Violence in Yemen's key port city of Hodeidah is at its worst in months, the United Nations humanitarian coordinator has warned, as a leader of the Houthi rebels vowed that his side would carry on fighting despite US calls for a ceasefire.

Hodeidah, a vital entry point for UN and other humanitarian aid, has become the center of the conflict in Yemen between US-backed Arab allies led by Saudi Arabia and Houthi rebels backed by Iran.

Fighting has escalated since US Secretary of State Mike Pompeo and Secretary of Defense Jim Mattis a week ago called on all participants in Yemen's civil war to agree to a ceasefire in the next 30 days and start peace talks.

"The uptick in violence is some of the fiercest fighting we've seen in months and months," UN humanitarian coordinator in Yemen, Lise Grande, told CNN.

The Saudi-led coalition claimed Monday it had made significant gains in the battle to capture Hodeidah from the Houthis.

Read More