The Saudi-led coalition says it has freed 200 Houthi prisoners to support peace efforts to end the nearly five-year war in Yemen.

The coalition said in a statement published by Saudi state media on Tuesday that it would ease restrictions on Yemeni airspace to allow flights from Sanaa, which is controlled by the Houthis, to transport those in need of medical treatment abroad.

The coalition intervened in Yemen in March 2015, confronting the Houthi group that removed the internationally recognized government from power in Sana'a.

The decision to release the prisoners came about two months after a relative halt to Houthi attacks against Saudi Arabia, and shortly after a Saudi official announced that the kingdom had opened a "channel of communication" with the Houthis to end the war.

However, a recent escalation has worried UN officials, with military confrontations in Hodeida following the Houthis' announcement of drones and ballistic missiles, killing and wounding more than 350 Yemeni soldiers, including Saudis, Emiratis and Sudanese.

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In return, coalition warplanes launched air strikes on Houthi positions in Hodeidah and the west coast.

The United Nations on Tuesday urged all parties to the Yemeni conflict to "refrain from any action that might run counter to the provisions and spirit of the Stockholm Agreement (signed at the end of 2018), and to avoid further escalation of the situation" in Hodeida, west of the country.

The head of the UN mission in support of the agreement Hodeidah Abhijit Guha, in his statement on Tuesday, "deeply concerned about the escalation of violence in the province of Hodeidah and surrounding areas in the past few days."

He said that "the increase in the number of air strikes carried out during the past 72 hours, clearly contrasts with the relative calm after the establishment of control points."

The Houthis agreed with coalition-backed government forces during talks in Sweden last December to exchange some 15,000 prisoners.

But the deal has not been implemented, although at the end of September the Houthis released 290 government fighters on a unilateral initiative.

In January, the coalition released seven Houthi prisoners.