``This military escalation not only constitutes a violation of the Hodeida ceasefire agreement but it runs against the spirit of the ongoing UN-facilitated negotiations that aim to achieve a nationwide ceasefire, humanitarian and economic measures and the resumption of the political process,'' said U.N. Special Envoy for Yemen Martin Griffiths in a statement.

Griffiths said he is following with ``deep concern'' reports of civilian casualties, including women and children.

¨I have been engaging with all sides. I call on them to immediately stop the fighting, respect the commitments they made under the Stockholm agreement,`` he said.

In 2018, heavy fighting erupted in Hodeida after government forces backed by a Saudi-led coalition moved in to wrest control of the strategic city from the Houthis.

After month of clashes, the warring sides signed a U.N.-brokered agreement in Sweden in December 2018 that included a cease-fire in the port city and an exchange of more than 15,000 prisoners. The deal, seen as an important first step toward ending the conflict, was never fully implemented.

On Monday, the U.N. mission to Yemen called for an immediate halt of violence in order ``not to put the population at further risk and jeopardize the delivery of humanitarian assistance.''

The conflict in Yemen began with the 2014 takeover of the capital Sanaa by the Iranian-backed Houthis. A Saudi-led coalition allied with the government has been fighting the Houthis since March 2015.

The war in Yemen has spawned the world's worst humanitarian crisis, leaving millions suffering from food and medical shortages. It has killed over 112,000 people, including fighters and civilians, according to a database project that tracks violence.