UN human rights chief Michelle Bachelet says her office has credible reports of serious violations by the Taliban.
They include summary executions, blocking girls from attending school and recruitment of child soldiers.
Though the ultraconservative Islamists have sought to present themselves as more moderate since taking control of the country, evidence gathered by the UN suggests otherwise.
The Taliban practised a strict version of Islamic law (Sharia) when they ran Afghanistan before 2001.
They retook the country nine days ago, with the fall of the capital, Kabul.
Since then, the militants have tried to convey a more restrained image, promising rights for women and girls and some freedom of speech.
Ms Bachelet says women’s rights are a “fundamental red line” and has called on UN member states to create a dedicated body to monitor human rights in Afghanistan.
China’s UN envoy said the US military and other international forces should be held accountable for rights violations committed during their time in Afghanistan.
Thousands of Afghans are still massing at Kabul airport, hoping to flee the country before 31 August. That is the deadline set by US President Joe Biden for American troops to leave Afghanistan.
The UK, France and Germany have asked for an extension.
Even before the Taliban retook control, more than 550,000 people had been forced to flee their homes this year due to fighting, according to the UNHCR, the UN refugee agency.
Meanwhile, the head of the US Central Intelligence Agency (CIA) held a secret meeting in Kabul with Taliban founder Mullah Baradar, the Washington Post reported on Tuesday.
If confirmed, it would be the highest-level meeting so far between the US and the Taliban since the fall of Kabul and the removal of the US-backed government.