"States may shut my office out, but they will not shut us up; neither will they blind us," UN rights chief Zeid Ra'ad Al Hussein said, opening the 32nd session of the UN Human Rights Council.
He decried "an emerging pattern" in which a growing number of states were refusing access to his staff and other UN representatives tasked with investigating allegations of rights violations in their countries.
Zeid voiced alarm at the situation in war-ravaged Syria, where no UN human rights monitors have been allowed in since the deadly conflict erupted in March 2011.
"Words cannot convey how profoundly I condemn this situation," he said.
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Zeid also criticised Venezuela, which for the past two and a half years has refused to even issue a visa to his representative in the region.
"Its comprehensive denial of access to my staff is particularly shocking in the light of our acute concerns regarding allegations of repression of opposition voices and civil society groups," he said.
Iran meanwhile had blocked all access to his staff since 2013, which he said was "particularly regrettable given the reports we continue to receive of fundamental problems with the administration of criminal justice" in the country.
Especially worrying, is the "continued execution of large numbers of people, including juveniles", he said.
Belarus, Eritrea and North Korea have also flatly refused all access by UN rights monitors, while Israel has repeatedly refused to cooperate with probes into the situation in the Occupied Palestinian Territories, he said.
Zeid stressed that rights investigators would not turn a blind eye to situations simply because they were blocked from investigating them on the ground.
"If access is refused us, we will assume the worst," he said.
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