Sudan has deported 30 Eritreans, including at least six registered as refugees, back to their homeland where they risk detention and abuse, Human Rights Watch (HRW) said on Thursday.
Sudan's ministry of foreign affairs could not immediately comment, and a source at the country's refugee commission had no information.
The HRW statement came the same day that Eritrea's President Issaias Afeworki landed in Khartoum for a three-day official visit.
The Eritrean group of 30 was arrested near the Libyan border in early February and held for three months without charge and without access to the United Nations refugee agency (UNHCR), HRW said.
"On May 1, 2014, Sudanese authorities in eastern Sudan handed 30 Eritreans over to Eritrean security forces," the group said, citing information from two advocates who were in telephone contact with the group.
Opposition activists from Eritrea say the number deported was 45.
"Sudan is forcibly returning Eritreans to serious risk of detention and abuse at the hands of a brutal government," said Gerry Simpson, senior refugee researcher at Human Rights Watch.
The watchdog said international law forbids countries from deporting asylum seekers without first allowing them to apply for asylum and considering their cases.
International law also prohibits deportation to places where they would face a risk of death or ill-treatment, HRW added.
According to the UNHCR, an average of 600 refugees from Eritrea make their way to neighbouring Sudan each month.
Many Eritreans who enter Sudan travel on to Israel or try to reach Europe, often with the help of smugglers.
Sudanese officials said last week that troops from Sudan and Libya had rescued hundreds of illegal migrants in the scorching desert border region between the two countries where traffickers had dumped them.
Ten migrants died, according to Sudanese authorities. Troops escorted the survivors, some of them in ill health, to the northern Sudanese town of Dongola.
A source familiar with the situation told AFP on Thursday that the death toll was higher than 10, and that there had been an exchange of fire between Sudanese troops and the smugglers.
Troops initially found about 300 hungry and thirsty victims. They later came across even more, bringing the total number rescued to 600, the military said.
An AFP journalist said most of the migrants who reached Dongola appeared to be Ethiopian or Eritrean, but there were some Sudanese as well.
HRW, citing information from two Eritreans in the Dongola group, said a court in the town convicted the migrants of immigration offences.
This puts them too at risk of deportation, HRW said.
A UNHCR spokesman could not be immediately reached for comment on Thursday.
UNHCR does not currently have access to the northwest border region, the agency's Sudan representative, Mohammed Adar, told AFP last week.
But he said the Sudanese government "has been relatively respectful of our non-return advisory to Eritrea and deportations to Eritrea have been rare."
Adar added that UNHCR did not expect Eritreans among the newly-rescued group to be sent back.