A court in Ethiopia has sentenced two Swedish journalists to 11 years in prison on charges of supporting terrorism after they illegally entered the country with a Somali rebel group.

Judge Shemsu Sirgaga ruled that Johan Persson and Martin Schibbye, both freelances, should suffer "rigorous imprisonment" following their convictions last week.

Ethiopian troops captured Persson, 29, and 31-year-old Schibbye six months ago. They were detained during a clash with rebels in Ogaden, eastern Ethiopia's ethnic Somali region, where there has been a fight for independence since the 1970s.

The journalists claimed they were investigating alleged human rights abuses and had been using the Ogaden National Liberation Front (ONLF) rebel group as guides in a region the Ethiopian authorities will not allow journalists to enter. Ethiopia recently designated the ONLF as a terrorist organisation.

The judge accused the ONLF of organising the Swedes' journey, starting in London. There was no visible reaction to the sentencing from the convicted men.

Speaking from Stockholm, Karin Schibbye, Martin's mother, said: "It's absurd. You can't really take in that they are sentenced to 11 years. It's obviously so wrong.

"They are innocent. They entered the country illegally and should be punished for that and nothing else. At least it was not the maximum sentence of 18 and a half years, which is a good sign of how the process could develop."

The 64-year-old said she was allowed no phone calls or letters to her son, but has been able to visit him in prison after obtaining a permit from the Swedish embassy. Conditions were harsh, she said, adding: "It is not a nice place. Their dormitory is overcrowded, with 70 beds for 200 inmates.

"In the first weeks, they had to sleep on the floor with rats, snakes, bugs. Now their situation has improved and they are sharing a bed. They pay a local restaurant to bring them lunch and dinner every day. The embassy also visits and brings food.

"They get support from other prisoners, who say: 'We are criminals, you are not.' There are a lot of abuses there, but they are secure and have had high-ranking prisoners looking after them from day one."

Persson and Schibbye are both contributors to the Sweden-based Kontinent photojournalism agency, while Schibbye is also a writer. The two regularly had their work published in national newspapers in Sweden and Norway.

The pair have acknowledged that they entered Ethiopia illegally. They said they had been gathering news about a Swedish oil company exploring Ethiopia's Somali region for oil and had pitched the story to Filter, a Swedish reportage magazine.

Mattias Göransson, Filter's editor-in-chief, said that Persson had previously visited the Dadaab camp in Kenya, where he heard claims from refugees about Ogaden's oilfields.

"They wanted to go to Ogaden to check if the claims were correct. They wanted to see for themselves," he said.

Asked what had gone wrong, Göransson – who testified for the defence in the men's trial – said: "I don't know. I don't know if we will ever know."

He speculated that an informant may have tipped off the Ethiopian authorities, and also noted that an unknown person had hacked into Schibbye's website a few weeks before his arrest.

This year, Ethiopia has detained more than 150 people, including reporters, in a crackdown the opposition says is designed to stifle moves towards more democracy. The government denies the accusations.

The guilty verdicts have provoked anger in Sweden and suggestions in the media the case has taken on a political dimension. Swedish government officials have said that the two Swedes were on a "journalistic mission", and have pushed for them to be freed. Anders Jorle, a foreign ministry spokesman, called the sentencing expected but regrettable. He said Sweden had been in contact with the Ethiopian government over the court case "at a high level".

The men's lawyer, Thomas Olsson, told the national broadcaster SVT that the sentence was a disappointment, but that he and his clients had not yet decided whether they would appeal.

"We will try to adjust to the new situation and help them as much as possible," he said, noting that an appeal could take up to two years and that the alternative would be for them to try to get pardoned.

"The latter means that they have to confess to these crimes and apologise – which, of course, is a hard thing to do when you regard yourself as innocent," he said.

Amnesty International called for the pair to be released immediately and unconditionally. Claire Beston, its Ethiopia researcher, said: "There is nothing to suggest that the two men entered Ethiopia with any intention other than conducting their legitimate work as journalists.

"The government chooses to interpret meeting with a terrorist organisation as support of that group and therefore a terrorist act."