SANAA, Yemen (AP) — 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 hospitals overwhelmed with patients, 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 backing the internationally recognized government and allied forces began launching airstrikes against the Houthis in March.

The Houthis have maintained a months-long siege of Taiz, while aid groups and residents have been complaining that Houthi indiscriminate shelling into the city is killing civilians on a semi-daily basis.

Residents in the city of some 250,000 say more than 1,600 civilians have been killed in the siege since March, largely by Houthi shelling.

Taiz is considered Yemen's cultural capital, with one of the country's main universities and a relatively cosmopolitan, politically minded population. It is also a pivotal frontline in the war, lying on the border between northern and southern Yemen.

On Friday, shelling by Houthi forces killed three civilians in the city, according to independent security officials. There were also clashes elsewhere.

In Marib province, at least 27 fighters from both sides were killed in heavy fighting, according to independent security officials. Meanwhile, more than 30 people were killed over the past 24 hours on a front line between the provinces of Taiz and Lahj, the officials said.

Also, the Saudi-led coalition targeted Houthi locations northeast of the capital, Sanaa, they said. Eight Houthis and allied fighters were killed by airstrikes in Sanhan town in Sanaa province. The airstrikes also targeted weapons depots in those locations.

All officials spoke on condition of anonymity because they were not authorized to talk to the media.