SANAA, Yemen — 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 international aid group Oxfam said humanitarian organizations received warnings over the weekend for staff to evacuate Hodeida by Tuesday ahead of an offensive.

“It’s also our plan and intention to stay and deliver,” Lowcock said. “We have dozens of U.N. staff still in Hodeida. We are working with very largely numbers of Yemeni organizations and individuals.”

The United States urged all parties of the conflict to ensure humanitarian access to the Yemeni people, Secretary of State Mike Pompeo said Monday. The US, he said, is closely following developments in Hodeidah and urged Emirati leaders to preserve “the free flow of humanitarian aid and life-saving commercial imports.”

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.

The U.N. Security Council, which was also briefed by Griffiths via video from Amman, strongly backed his efforts and Lowcock’s and stressed again that “only negotiated settlement can bring the war to an end.”

Russia’s U.N. Ambassador Vassily Nebenzia, the current council president, said discussions centered on “the need for de-escalation.”

“We are hoping for the efforts of the special envoy to bring positive results on it, and we left it in his hands for the time being,” Nebenzia said.

International aid group Doctors Without Borders said Monday the Saudi-led coalition attacked a cholera treatment center in the northern province of Hajja.

The group, known by its French acronym MSF, has temporarily frozen its activities in the area, “until we guarantee the safety of our staff and patients,” said João Martins, MSF’s head of mission in Yemen on Twitter.

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.

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Associated Press writer Edith M. Lederer at the United Nations contributed to this report.

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