The United States announced a resumption of aid to Yemen's rebel-held north on Friday to fight a looming famine as the country's nearly six-year-old war grinds on. U.N. officials warned that a blockade of fuel deliveries to a main port was heightening the world's worst humanitarian crisis.

The aid concern came as President Joe Biden's envoy to Yemen expressed frustration at the country's Houthi rebels, saying they were focusing on fighting to capture more territory while an international and regional diplomatic push was underway to end the conflict.

"Tragically, and somewhat confusingly for me...