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.

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.

U.N. Secretary-General Antonio Guterres said Monday that there has been a recent lull in the fighting and that Martin Griffiths, the U.N. envoy for Yemen, "is shuttling between Sana'a and also the UAE and Saudi Arabia to hope that there will be a way to avoid the military confrontation in Hodeidah."

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 tweeted.

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.

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