A leading opposition figure in Niger who returned from self-imposed exile last week was jailed on Monday to serve out the rest of his time for a conviction of baby smuggling, his family told AFP.

Hama Amadou, 69, who had flown home last Thursday after his mother died, "was incarcerated early today in Filingue," 180 kilometres (110 miles) north of the capital Niamey, a relative said.

"He went to court voluntarily and was taken to prison."

A former premier and parliamentary speaker, Amadou has been nicknamed "the Phoenix" for his political comebacks.

In November 2015, he was arrested on his return from abroad and charged with baby-trafficking -- a case that supporters said was trumped-up.

He campaigned for the March 2016 presidential elections from behind bars, gaining 17.7 percent of first-round votes behind President Mahamadou Issoufou, who won 48.4 percent.

Amadou campaigned for the 2016 presidential elections from his prison cell
Amadou campaigned for the 2016 presidential elections from his prison cell AFP / ISSOUF SANOGO

He was released on medical grounds four days before the runoff ballot, flew to France and remained abroad until his return last week.

In March 2017, 20 people, including Amadou, were convicted in the case, which allegedly entailed smuggling babies from Nigeria via Benin for wealthy couples in Niger.

He was given a 12-month sentence in absentia, of which eight months have still to be served, after time spent in prison is taken into account. One of his wives was also sentenced.

In August this year, Amadou was named by the Nigerien Democratic Movement (Moden), the country's main opposition party, to be their champion in the next presidential elections, due in December 2020.

Issoufou, a former prime minister, has been president since 2011.

His country has a history of political volatility and has only had a multi-party democracy since 1990.

The impoverished Sahel state has one of the lowest rates of development in the world. Three-quarters of the population live on less than $2 a day.