BRUSSELS (Reuters) - The European Union has asked Greece to move migrants most at risk of contracting the coronavirus from overcrowded camps on its Mediterranean islands, the EU's top migration official told Reuters on Tuesday.

Home Affairs Commissioner Ylva Johansson said Athens had opposed moving the migrants to the Greek mainland, citing the absence of coronavirus cases in the camps while the disease is spreading elsewhere in Greece.

With much of Europe in lockdown under measures to curb the spread of the virus, Johansson said the elderly, sick and others most at risk of infection could be moved to other areas on the Greek islands.

"We are working together with the Greek government and the Greek authorities to agree on an emergency plan to help reduce the risk as much as possible in the overcrowded hotspots on the islands," Johansson said in a telephone interview.

"It could be relocation of the most vulnerable individuals from the overcrowded camps to other areas on the islands."

The situation in the camps is dire at the best of times. They are designed for several thousand people but the European Commission, the EU executive, says they house about 42,000.

Rights groups frequently sound the alarm about conditions that aggravate the suffering of people mainly fleeing war and poverty in the Middle East and Africa, and looking for better lives in Europe.

The situation has become even more tense since Greece last week imposed restrictions on the movement of people living in the camps to combat the spread of the coronavirus.

"Thousands of people, including older people, those with chronic diseases, children... pregnant women, new mothers, and people with disabilities, are trapped in dangerously overcrowded, deplorable conditions on the islands amid the COVID-19 pandemic," Human Rights Watch said on Tuesday, referring to the illness that the coronavirus can cause.

"Forcing asylum seekers to remain in conditions that violate their rights and are harmful to their well-being, health, and dignity cannot be justified on grounds of public health," the New York-based group said in a statement.

Greece has said it receives insufficient EU help in caring for the migrants arriving across the Mediterranean from Turkey, which has recently allowed more refugees and migrants, many of them from Syria, to leave its territory for Europe.

To assuage both Athens and Ankara, the EU promised earlier this month to take in some 1,600 unaccompanied children from the camps. Johansson said that has not happened yet and that Greek authorities still needed to identify the minors.

"My hope is that we can start with the relocation in the coming weeks," she said.