(CNN) At least 80 Yemeni soldiers attending prayers at a mosque were killed and 130 others injured in ballistic missile and drone attacks by Iran-backed Houthi rebels, the UN Special Envoy for Yemen reported Sunday.

"I have said before that the hard-earned progress that Yemen has made on de-escalation is very fragile. Such actions can derail this progress," Martin Griffiths warned, urging all parties to "stop the escalation" and focus energies into policy instead.

The attack happened in the province of Marib on Saturday at a mosque at a military camp, in what Yemen's President Abdu Rabu Mansour Hadi condemned as a "heinous" and "cowardly terrorist operation committed by the Houthis."

Yemen has been embroiled in a yearslong civil war that has pitted a coalition backed by Saudi Arabia and the United Arab Emirates against the Iran-backed Houthi rebels.

Yemen's Ministry of Defense said the attack was "to avenge the killing of the Iranian terrorist Qasem Soleimani," who died in a US drone strike in Iraq on January 3. The ministry offered no evidence to show how it might know the rebels' motive.

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