The U.N. envoy for Yemen reported “significant progress” in negotiations toward a nationwide cease-fire in the war-torn country on Thursday, but warned of stark challenges as coronavirus spreads at an unknown rate across the Arab nation.

Martin Griffiths urged Yemen’s warring sides — the internationally recognized government and Houthi rebels who control the capital and most of the country’s north — to quickly resolve their differences over humanitarian and economic measures needed to move peace efforts forward and help the country counter the virus.