Acting High Commissioner for Human Rights Bertrand Ramcharan "has asked the Uzbek government not to carry out the execution of detainees who have appealed their convictions to the United Nations Human Rights Committee," Ramcharan's office said in a statement.
In his March visit to the former Soviet Central Asian republic, Ramacharan had "underlined the importance of respecting such interim measures of protection," the statement read.
On Tuesday, Amnesty International added its voice to the concern saying it "has learnt of the execution of four men who were executed despite the intervention of the Human Rights Committee (and) unconfirmed reports that two more were also executed in recent weeks."
Muzaffar Mirzaev, 29, was the latest person to have been secretly executed in June "despite allegations that he was mentally ill and an intervention by the United Nations Human Rights Committee," Amnesty International said.
But his relatives fear that another prisoner who has appealed to the committee, Nodirbek Karimov, 22, "may have been executed or could be executed at any time," the statement said.
Visiting Uzbekistan in December, the US's expert on torture Theo van Boven said that torture by the Uzbek security forces appeared to be "systematic".
Getting the authoritarian yet secular leadership of President Islam Karimov to comply with the UN has been difficult.
Uzbekistan is an ally in the United States' anti-terrorism campaign and continues to host US-led forces who overthrew neighbouring Afghanistan's Taliban government in 2001.
But critics have charged that Karimov's rule may actually encourage individuals to carry out extremist acts as there is virtually no legal way for them to express dissent.
Activists put the number of political and religious prisoners in Uzbekistan at around 6500.