The U.N. envoy to Yemen said he was "alarmed" over the escalation of violence after a rebel drone attack on the country's largest airbase killed six loyalist soldiers. In tweets posted overnight Thursday, Martin Griffiths urged all parties to Yemen's protracted conflict to exercise restraint. Griffiths tweeted that he was "alarmed by [Thursday] escalation of violence in Yemen." He urged "all parties to the conflict to exercise restraint and refrain from further escalation" and to "create a conducive environment to maintain the positive momentum generated" in Sweden.

The Shiite Houthi rebels said they carried out the strike, which hit a military parade at Al-Anad air base, in government-held Lahij province some 60 kilometers (40 miles) north of Yemen's second city Aden. Six loyalist soldiers were killed and at least 12 people wounded, including top commanders, medics said.

The attack comes as the U.N. tries to get peace talks going between the Houthis who control northern Yemen and the Saudi-backed government of Abed Rabbo Mansour Hadi by overseeing a limited cease-fire in the Red Sea port city of Hodeidah. The cease-fire only applies to Hodeidah province, but the British Ambassador to Yemen, Michael Aron, tweeted on Thursday that an escalation anywhere in Yemen "goes against the spirit of the Stockholm agreement."

The U.N. was hoping last month's talks in Sweden would help launch formal peace talks between Yemen's warring parties. Thursday's attack is likely to create a new obstacle for those efforts.

Implementation of the deal, the first breakthrough in peace efforts in five years, has stalled as the sides disagree on who will control Hodeidah after the withdrawal. Hodeidah is of strategic importance because of its port, which is the entry point for some 80 percent of Yemen's imports and aid. In June, a Saudi-led coalition supporting the Yemeni government began a major offensive to seize Hodeida, which has been under the control of rebels since October 2014, a month after the Houthis overran the capital Sanaa. Yemen lies beside the southern mouth of the Red Sea, one of the most important trade routes in the world for oil tankers. It has been wracked by conflict for the last five years when Shiite Houthi rebels overran much of the country. Saudi Crown Prince Mohammed bin Salman, the former Saudi defense minister, and Saudi Arabia's allies launched Operation Decisive Storm in March 2015. Civilians have borne the brunt of the conflict. The last available U.N. figure for the civilian death toll was published in 2016 and stood at more than 10,000. The Armed Conflict Location & Event Data Project, which tracks violence in Yemen, puts it at around 57,000 people.