Turkey's upcoming presidential elections will not be credible unless the country lifts restrictive emergency measures imposed following a failed 2016 coup, the United Nations human rights chief said Wednesday.

‘Over the past two years, through successive states of emergency, the space for dissent in Turkey has shrunk considerably,’ Zeid Ra'ad Al Hussein said in a statement, noting a crackdown on media criticism and freedom of assembly.

‘It is difficult to imagine how credible elections can be held in an environment where dissenting views and challenges to the ruling party are penalised so severely,’ Zeid added.

President Recep Tayyip Erdogan, in power for the last 15 years as premier and now president, will seek a new term in the June 24 vote.

‘Elections held in an environment where democratic freedoms and the rule of law are compromised would raise questions about their legitimacy, and result in more uncertainty and instability,’ the UN rights chief said.

The state of emergency was introduced in Turkey five days after the attempted putsch on July 15, 2016.

More than 1,300 associations and foundations have been shut down under the measures.

In addition, more than 140,000 public sector employees have been sacked or suspended including judges over alleged links to putschists or Kurdish militants.

Meanwhile, some 50,000 people have been taken into custody on terror charges.