The World Health Organization is calling for access to Taiz, Yemen's third largest city which has been under siege for months amid a civil war between an internationally recognized government and Shiite rebels.

The organization said in a Thursday statement that five of its trucks have been prevented from entering the city since Dec. 14.

It said the trucks carried "health supplies that urgently needed to be delivered" to overwhelmed hospitals, including trauma medicines and "500 cylinders of oxygen that are critically needed by the hospitals."

Yemen has been embroiled in conflict since Shiite rebels known as Houthis took over the capital, Sanaa, in September 2014. The conflict escalated when a Saudi-led coalition began launching airstrikes against the Houthis in March.

The Houthis have maintained a months-long siege of Taiz.