Rwanda's cabinet has decided to synchronise the dates for its parliamentary and presidential elections, the prime minister's office said in a statement.

At a meeting on Friday chaired by President Paul Kagame, ministers approved his proposal to amend the constitution, "allowing for the harmonisation of parliamentary and presidential election calendars", it said.

The polls are now expected to take place in August next year, the date originally set for the presidential election, effectively delaying the parliamentary ballot that had been scheduled for September 2023.

The move -- which still needs parliamentary approval -- was unveiled in a statement that also confirmed that Kagame critic Paul Rusesabagina's 25-year sentence on terrorism charges had been commuted by presidential order.

Rusesabagina, 68, was released late Friday to the Qatari ambassador's residence in Kigali and he is expected to stay there for a couple of days before flying to Doha and then to the United States, a  US official said.

While Rwanda lays claim to being one of the most stable countries in Africa, rights groups accuse Kagame of ruling in a climate of fear, stifling dissent and free speech.

Kagame became president in April 2000, although the former rebel chief has been regarded as de facto leader since the end of the 1994 genocide.

He was returned to office -- with more than 90 percent of the vote -- in elections in 2003, 2010 and 2017. His most recent victory followed controversial constitutional amendments that allowed him to serve a third term and could see him rule until 2034.

The move to harmonise the elections was originally floated in February by the new head of the National Electoral Commission, Oda Gasinzigwa, who is controversially a senior member of Kagame's ruling Rwandan Patriotic Front (RPF).

NEC secretary general Charles Munyaneza told local media the change would save money.

"We no longer depend on donor funding to conduct our elections. And when we look at our elections, be it the parliamentary elections or the presidential elections, you find that each consumes around seven billion Rwandan francs (about $6 million)," he said.

"Yet about eight billion francs ($7 million) would be enough if they were held simultaneously."

The opposition, which has only meagre representation in parliament after the RPF and smaller allied parties swept the 2018 legislative election, also welcomed the move.

"It will be cost effective both for the state and for political parties," Frank Habineza, leader of the Democratic Green Party of Rwanda, told AFP.

"So, if both are held at the same time the state will be able to save taxpayers' money which could be used to construct more roads, more schools or hospitals."