SANAA, Yemen (AP) — Heavy fighting in Yemen between pro-government forces and Shiite rebels has killed more than 600 people on both sides in recent days, security officials said Monday.

Government forces, backed by a Saudi-led coalition, have been advancing along the western coast in recent weeks as they battle the Iran-allied rebels, known as Houthis. The fighting has escalated as government forces close in on the Red Sea port of Hodeida, a vital lifeline through which most of Yemen’s food and medicine enters.

The officials spoke on condition of anonymity because they were not authorized to brief the media. Witnesses, who spoke on condition of anonymity for fear of reprisals, said the fighting has forced dozens of families to leave their homes.

U.N. Secretary-General Antonio Guterres said Monday that U.N. envoy Martin Griffiths was engaged in “intense negotiations,” shuttling between Yemen’s capital Sanaa, controlled by the Houthis, and Saudi Arabia and the United Arab Emirates to avoid a “military confrontation in Hodeida.”

U.N. humanitarian chief Mark Lowcock told reporters after briefing the Security Council behind closed doors later Monday that it’s critical to prevent “a battle” for Hodeida which is a vital link for supplying millions of Yemenis with the necessities of life.

He said “90 percent of food, fuel and medicines in Yemen are imported” — and 70 percent come through Hodeida, including desperately needed humanitarian aid for over 7 million people.

The United Nations warned Friday that a military attack or siege on Hodeida would affect hundreds of thousands of civilians. Some 600,000 people live in and around the city.

Guterres, the U.N. chief, said there has been a recent “lull in the fighting” to allow for discussions and hopefully avoid a battle.

Yemen has been embroiled in a civil war pitting the coalition against the Iran-backed Houthis since March 2015. The coalition aims to restore the government of self-exiled President Abed Rabbo Mansour Hadi.

The three-year stalemated war has killed more than 10,000 people and displaced more than 3 million. It has damaged Yemen’s infrastructure, crippled its health system and pushed the Arab world’s poorest country to the brink of famine.

The U.N. considers Yemen to be the world’s worst humanitarian crisis, with more than 22.2 million people in need of assistance. Malnutrition, cholera and other diseases have killed or sickened thousands of civilians over the years.