The aid group Doctors Without Borders says a projectile from an unknown source has hit a hospital it supports in Yemen's northern Saada province, killing four and injuring 10.

The group, known as MSF, says in a statement that the Sunday attack led to "the collapse of several buildings of the medical facility."

Of those injured, three are MSF staff, and two are in critical condition, it said.

MSF operates in eight Yemeni provinces at a time when many foreign aid groups and even United Nations personnel have been evacuated.

Shiite rebels known Houthis took over the capital, Sanaa, in September 2014; in March, a Saudi-led coalition started launching airstrikes against Houthi and allied forces' positions.

Saada, a Houthi stronghold, has faced a particularly intense bombardment.