By Sushil Sharma BBC correspondent in Kathmandu

The United States has called for the United Nations to intervene in resolving the long-running Bhutanese refugee crisis in Nepal.

The US ambassador to Nepal, Michael Malinowski, said the UN refugee agency (UNHCR) was uniquely qualified in handling refugee repatriation.

About 100,000 refugees from Bhutan live in camps in Nepal

His comments came after Nepal and Bhutan carried out a controversial screening of the refugees ahead of a plan to repatriate them to Bhutan.

This is the first time that the US has called for the direct intervention of the UN refugee agency in the repatriation of more than 100,000 Nepali-speaking refugees.

They have been living in eastern Nepal following their alleged eviction from Bhutan 13 years ago.

In a rare letter to Nepalese newspapers on Thursday, Mr Malinowski stressed that the UNHCR could ensure a just solution based on what he said were internationally accepted standards.

The move follows the recent screening exercise to determine the refugees' nationality.

It found that less than three percent of the refugees were bona fide Bhutanese citizens and had the right to return to Bhutan.

More than 70% were described as voluntary migrants whose repatriation would depend on the renewal of their citizenship by the Bhutanese Government, which had earlier refused to take them back.

Human rights call

International human rights groups including Amnesty International and Human Rights Watch have joined refugee groups in urging Bhutan's donors to reject the categorisation, saying that it could render thousands of refugees stateless.

The rights groups also called for the UNHCR's intervention to facilitate and monitor the refugees' verification and repatriation process.

In an obvious endorsement of that view, American ambassador Michael Malinowski urged Nepal and Bhutan to work with the UNHCR, and pledged to assist in the resolution of the crisis that he said could be a lengthy and costly process.

The UNHCR says it can not intervene unless both parties ask it.