DUBAI-The United Nations envoy for Yemen scrambled Tuesday to revive a six-month truce after a missed deadline raised fears of a return to war.

Special envoy Hans Grundberg pledged “relentless efforts” to reinvigorate the truce, which lapsed on Sunday after bringing a sharp reduction in clashes since it came into force in April. Huthi rebels have been fighting pro-government coalition since 2015, in a war that has left hundreds of thousands dead and created the world’s worst humanitarian crisis according to the United Nations.

Grundberg’s plan to extend the truce -- which was initially set to last two months and was renewed twice, to half a year -- and to broaden it to new areas of agreement was rejected by the Huthis.