TEHRAN/SEOUL - Iran said on Tuesday that it had rejected fresh requests by Ahmed Shaheed, the UN's monitor for human rights in the Islamic republic, to assess the situation on the ground.

"We unfortunately do not consider Ahmed Shaheed to be an impartial rapporteur," foreign ministry spokesman Abbas Araqchi said during his weekly briefing with reporters in Tehran, Iranian media reported.

Araqchi said Tehran had received two official letters from Shaheed requesting access to assess the human rights situation in the country.

But such a visit, he said, would not happen as long as Shaheed "continues his trend of writing unfair reports about Iran."

Tehran has so far refused to let Shaheed visit the country since being appointed as the UN's monitor for human rights in Iran in 2011, and has responded to only a handful of official requests for information.

Shaheed has thus far relied on contacting campaigners and victims inside Iran, as well as exiles and human rights groups, to write his reports on the country.

In March, the UN body monitoring human rights in Iran sounded the alarm after Shaheed's latest report touched on restriction of freedom of speech and a slew of other abuses, including torture, forced confessions, secret executions and the jailing of members of the political opposition.

At the time, he also condemned an increase in the number of journalists arrested, pointing out that more than 50 reporters were behind bars, and said that the crackdown was meant to intimidate the press ahead of Iran's presidential elections in June.

Iran has accused Shaheed of failing to consider whether the international community was breaching the rights of its people by imposing sanctions over Tehran's controversial nuclear drive.

Meanwhile, a UN inquiry gathering harrowing testimony of human rights abuses in North Korea appealed Tuesday for access to the country, even as Pyongyang condemned its work as slanderous and provocative.

The three-member Commission of Inquiry chaired by retired Australian judge Michael Kirby has just wrapped up five days of disturbing hearings in the South Korean capital Seoul - mostly testimony from North Korean defectors.

As Kirby prepared to give a final press conference Tuesday, the North's official Korean Central News Agency (KCNA) launched a bitter attack on the UN panel, calling its witnesses "human scum" manipulated by the South Korean authorities.

The commentary said the commission's work would only set back recent progress towards engagement between North and South Korea after months of heightened military tensions.

Meanwhile, Iran's foreign ministry said Tuesday it would take time to resume ties with Britain severed in the aftermath of an attack on the British embassy in Tehran in 2011. "We received the letter from British Prime Minister" David Cameron who expressed the "desire for a resumption of ties," ministry spokesman Abbas Araqchi said.

But "these issues need time and negotiations on an expert level," he told a news conference, adding that "it must be decided whether their approach and behaviour have changed".

Britain ordered Iran's embassy in London to shut after closing its own in Tehran following the storming of the compound by hundreds of Islamist students in November 2011.

They were demonstrating in front of the British embassy to express anger over new Western sanctions adopted against Tehran over its disputed nuclear drive.

Britain was one of the first European countries to adopt sanctions against the Iranian central bank to put pressure on Tehran over its nuclear programme.

Since then, both sides have said they would like to restore the ties.

Oman now represents Iranian interests in Britain, while Sweden has taken on similar responsibilities in Tehran.