Belarus has moved dissident blogger Roman Protasevich and his girlfriend Sofia Sapega to house arrest a month after President Alexander Lukashenko’s regime sparked international outrage when it captured them after forcing down a Ryanair flight.

Officials in Minsk said Protasevich and Sapega had struck a plea bargain under which the couple had agreed to co-operate with investigators, “expose co-conspirators” and confess to protest-related charges under which they face up to 15 years in prison.

The office of Sviatlana Tsikhanouskaya, Belarus’s exiled opposition leader, said that the duo were still “hostages”.

“We are in touch with Roman’s parents — they are not told anything about their son, they are not allowed to communicate with him,” Tsikhanouskaya’s office said in a statement.

Franak Viacorka, an aide to Tsikhanouskaya, said Belarus was attempting to engineer an easing of new sanctions announced by the EU, US, UK and Canada this week. The restrictions target 86 individuals and organisations as well as the country’s potash and petroleum sectors, a crucial source of hard currency.

“We expect that under this sanction pressure the regime will be trying to exchange batches of prisoners for lifting sanctions. They are playing the same game as after the elections in 2006 and 2010, exchanging people for benefits,” he told the Financial Times.

“[The EU should] not trust a single word that Lukashenko says. Everyone has to be released unconditionally.”

EU leaders this week called for the “immediate release of all political prisoners and arbitrarily detained persons” in Belarus, including Protasevich and Sapega.

One senior EU diplomat said the offer from Minsk was dismissed by foreign ministers this week because they did not see it as a serious proposition. “The move by Lukashenko’s regime was ignored,” the official said. “The only thing the EU will consider discussing is an immediate release of all 500 political prisoners and fair elections.”

Protasevich’s arrest is part of Belarus’s brutal crackdown on dissent since huge protests against Lukashenko’s dubious election win last year. About 35,000 Belarusians have been arrested, several of whom claim they were tortured in custody.

As the former editor of Nexta, the most prominent opposition channel on messaging app Telegram, Protasevich is accused of organising mass riots, and faces up to 15 years in jail if convicted. Sapega has been accused of causing unrest.

Belarus has paraded the dissident blogger, clearly under duress, at a series of interviews and press conferences where he has praised Lukashenko. On Friday, state media published what it said was footage of Protasevich and Sapega going for a walk in downtown Minsk.

Sapega’s lawyer Anton Gashinsky told Russian newswires she had been allowed to meet her parents for lunch in a Minsk café on Thursday, but was trailed by security services and not allowed to use the phone or internet.

Gashinsky added that Sapega, a Russian citizen, owed her release to Lukashenko’s meeting with Russian president Vladimir Putin after the Ryanair incident, and described her house arrest as a “very positive moment that will help find a positive solution to her fate in the near future”, according to Interfax.

The Kremlin said it would “continue to protect [Sapega’s] lawful interests”.

The unexpected move from jail came a day after the EU formalised sanctions against Belarus over the downing of the flight, which Ryanair called a “state-sponsored hijacking”.

A western diplomat said that moving Protasevich and Sapega to house arrest seemed to be part of an attempt to try to start negotiations with the west.

“It suggests there is a possible opening. It might be related to Russia, rather than the west, if Russia has told Belarus that it will have to bear the consequences [of the western sanctions] itself,” the diplomat said. “But to do this so late is quite stupid because once sanctions have been imposed, it is very difficult to get them revised — if that was the objective.”

Lukashenko, who has ruled Belarus with an iron fist for 27 years, claimed he deployed a fighter jet to escort down the Athens-Vilnius Ryanair flight in response to a fake bomb warning. He has also claimed the west is waging a “hybrid war” against Belarus.

Additional reporting by Michael Peel in Brussels