UN concerned by 'lack of transparency' in Jordan palace crisis
The Office of the United Nations High Commissioner for Human Rights said it was not yet clear whether Hamzah was still under de facto house arrest and said it seemed no charges had been brought.
Spokeswoman Marta Hurtado told reporters in Geneva that the rights office was following the situation, which erupted on April 3.
Hamzah, a former crown prince, has accused Jordan's rulers of corruption and ineptitude.
The Jordanian government had accused the prince - who was sidelined as heir to the throne in 2004 - of involvement in a conspiracy to "destabilise the kingdom's security" and arrested at least 16 people.
But King Abdullah said Wednesday that Hamzah, who has signed a letter pledging his loyalty to the sovereign following mediation by an uncle, was safe in his palace under his "care".
Hurtado said: "We are aware that there is an ongoing investigation but still it's not clear to us if following the mediation on April 5, Prince Hamzah is still under de facto house arrest or not.
"Aside from broad accusations, it appears that no charges have been yet brought and we are concerned at the lack of transparency around these arrests and detentions."
Hurtado said that any investigation, including one on the basis of national security, had to be conducted in line with international human rights law.
The arrests "were the latest in an escalating series of measures over the past year targeting, in particular, criticism of government policies and corruption", she added.