Security agents in Eritrea are hunting for Pentecostal Christians who attended an informal sermon given by an Ethiopian evangelist last month.

Preacher Surafiel Demssie had travelled to the capital, Asmara, on the first Ethiopian Airlines flight to Eritrea after the two countries ended their 20-year feud.

Eritrea only has four religions - Orthodox Christianity, Sunni Islam, the Roman Catholic Church, and the Evangelical-Lutheran church of Eritrea. Other religious groups are considered illegal.

So far, five of those who welcomed the preacher have been arrested in Asmara and others have gone into hiding fearing arrest.

One person, who is believed to have hosted the preacher, was arrested at the airport and taken away by security.

Security agents dressed in civilian clothes picked up some of those arrested.

"At first the police said they were arrested for blocking a road and would be released soon," said a relative of one of those arrested.

But later it transpired it was the National Security Agency who had arrested them, he said.

"We were very happy when the peace agreement was signed between Ethiopia and Eritrea," said Kesete Beraki, who advocates for the release of those in detention.

"We were hoping there would be changes in Eritrea too, but so far we have been disappointed."

Those arrested have not appeared before a court. Prisoners of conscience, including followers of evangelical churches, do not get charged formally before any court of law.

Human rights activist say hundreds of followers of these illegal churches are in detention.

The government banned Pentecostal churches in 2002 and several religious leaders have been detained incommunicado for more than a decade.