Guterres, speaking to reporters at U.N. headquarters, also accused the Taliban of breaking promises they made after seizing power in August to uphold the rights of women and girls, including allowing girls to attend school.
His comments underscored the urgent need for steps to ease the economic and humanitarian crises that have grown since the Taliban took power as the 20-year U.S. military intervention ended.
The Islamists' takeover saw billions in central bank assets frozen and international financial institutions suspend access to funds, although humanitarian aid has continued.
Banks are running out of money, civil servants have not been paid and food prices have soared.
"The crisis is affecting at least 18 million people - half the country's population," said Guterres, adding that a massive U.N. humanitarian aid operation is underway.
Guterres noted that the Afghan economy - kept afloat by foreign aid for two decades - was being buffeted by drought and COVID-19 before the Taliban seized power.