Human Rights Watch says Yemen's Houthi rebels are preventing vital aid from entering the besieged western city of Taiz.

In a statement Sunday, it says the rebels have for months restricted food and medical supplies to Yemen's third-largest city, in what it described as "serious violations of international humanitarian law."

It says the rebels are denying necessities to residents of Taiz and should "immediately end the unlawful confiscation of goods intended for the civilian population and permit full access by aid agencies."

Yemen's conflict pits a loose array forces fighting on the side of the internationally recognized government against the Shiite Houthis and troops loyal to a former president, who together control the capital, Sanaa, and large parts of northern Yemen.