The UN human rights agency warned on Monday that the human rights situation in Crimea had "significantly deteriorated" in the three-and-a-half years since Russia annexed the Black Sea peninsula from Ukraine.

The UN's report, presented in Geneva, chronicled alleged instances of arbitrary arrests and detentions, enforced disappearances, ill-treatment and torture, and at least one extra-judicial execution.

Read more: 'Authorities are step-by-step escalating repression' in Crimea

"There is an urgent need for accountability for human rights violations and abuses and for providing the victims with redress," said UN rights chief Zeid Ra'ad al-Hussein.

Lead author Fiona Frazer, who also runs the UN's office in Ukraine, told reporters that "a lack of impartiality of the judiciary" in Crimea had left those abused with little hope of legal justice and accountability.

Because investigators working on behalf of the UN High Commissioner for Human Rights were barred from entering Crimea, the report was based on interviews conducted on mainland Ukraine.

Prisoners deported from 'occupied territory'

According to the UN report, hundreds of prisoners had been illegally transferred from Crimea to prisons in Russia, while at least three detainees had died in custody after they were denied adequate medical treatment.

Read more: European Union extends Crimea sanctions by another year

Because Russia's annexation of Crimea from Ukraine in 2014 has never been internationally recognized, the Kremlin has effectively broken international law by transporting prisoners from an occupied territory.

Russia, however, maintains that its "incorporation" of Crimea was legitimized by a March 2014 referendum.

Ukrainian citizens' rights and culture also neglected

The UN also flagged Russia's citizenship laws imposed on Crimea residents as a human rights violation.

Immediately after annexing Crimea, Russian authorities said that all Ukrainians on the peninsula would be recognized as Russian citizens, unless they submitted a written rejection.

"Imposing citizenship on the inhabitants of an occupied territory can be equated to compelling them to swear allegiance to a power they may consider as hostile, which is forbidden under the Fourth Geneva Convention," Hussein said.

By May 2015, only about 100,000 people, or around 4 percent of Crimea's population, had rejected taking on Russian citizenship, according to the report. "Tens of thousands (of people) became foreigners and as a result face significant hardship," said Frazer. People left in citizenship limbo "cannot own agricultural land, vote and be elected, register a religious community, apply to hold a public meeting, hold positions in the public administration," she added.

Read more: UN committee urges Russia to end rights abuses in Crimea

Meanwhile, some 19,000 people, most of whom were civil servants wanting to keep their job under Moscow's rule, were in effect forced to renounce their Ukrainian citizenship.

Virtually all education in Ukrainian has also disappeared from the region, while authorities have also outlawed the Mejlis, a body representing Crimean Tatars, whose members have been subjected to intimidation, house searches and detention.

Frazer said that her office has continued to seek access to the Russian government through "official communication," although there was reportedly little sign of an imminent breakthrough.

As a permanent member of the UN Security Council, Russia can block any proposal seeking to hold its actions into account. Many western states have instead responded by imposing sanctions on Russia.

dm/kms (AP, AFP)