UNITED NATIONS — Pressure is mounting on the Saudi-led military coalition that seeks to stanch a rebellion in Yemen, as aid officials prepare to add Yemen to the ranks of the world’s most severe humanitarian crises and human rights groups point to what may be war crimes.

United Nations officials are expected to declare Yemen a so-called Level 3 — or most severe — humanitarian crisis, as the de facto military blockade on commercial ships restricts the supply of food and fuel into the Arab world’s poorest country, diplomats said Tuesday.

That is sure to complicate what is already a delicate diplomatic balance for allies of Saudi Arabia, including the United States, which are reluctant to even call it a blockade. The preferred term, as one United Nations Security Council diplomat put it, is a “controlled maritime area.”

Whatever it is called, its effects on civilians have been dire.

A fuel shortage has caused water pumps to stop functioning, heightening the risks of waterborne diseases. Hospitals are running out of medicines. An estimated 2.6 million children are at risk of contracting measles because vaccination programs have been interrupted, according to the United Nations children’s agency. And the death toll, according to figures released Tuesday by the World Health Organization, is more than 3,000.