The United Nations and more than 20 humanitarian groups are decrying a decision by a Saudi-led military coalition to block access to airports, seaports and land crossings into Yemen, saying it could bring millions of people closer to "starvation and death."

The groups including CARE, Save the Children and Islamic Relief, say about two-thirds of Yemen's population relies on imported supplies.

They say over 20 million people need humanitarian assistance, including 7 million facing "famine-like" conditions. Food supplies to feed them are set to run out within six weeks; vaccine supplies will last one month.

They called for the "immediate opening" of all air- and seaports in Thursday's statement.

The coalition initiated the blockade after Shiite rebels in Yemen fired a ballistic missile at the Saudi capital over the weekend.