Sudan has deported hundreds of Eritreans back to the hardline government they fled contravening international law, Human Rights Watch (HRW) said Tuesday.

Sudan is a key transit country for refugees hoping to reach Europe, and many of those sent back to Eritrea were arrested as they tried to cross from Sudan to Libya.

"Sudan is arresting and forcing Eritreans back into the hands of a repressive government without allowing refugees to seek protection," said Gerry Simpson from HRW, warning they could "face abuse" on their return.

According to the United Nations, around 5,000 Eritreans risk their lives every month to flee the country.

"International law also prohibits the deportation, return, or forced expulsion of anyone to a place where they face a real risk to their life, or of torture or ill-treatment," HRW said.

The deportations by Khartoum of at least 442 Eritreans — including six UN-registered refugees this month — comes as the European Union tries to work with Sudan to manage the tens of thousands hoping to cross the Mediterranean.

There was no immediate response from authorities in Khartoum or Asmara.

Eritrea, which borders Sudan, is accused of jailing thousands of political prisoners. Refugees from the repressive Red Sea state have in recent years made up one of the largest contingents of people risking the dangerous journey to seek a new life in Europe.

"The EU is in the early stages of working with Sudan and other African countries to tighten their border controls, tackle refugee and migrant smuggling, and improve the lives of potential migrants in those countries," HRW said.

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"If Sudan wants to market itself as a refugee-rights-respecting nation, it's going about it the wrong way."

Those who escape Eritrea describe crawling under razor wire, tiptoeing across minefields or sneaking past armed border guards in their bid for freedom.