Twenty-one schoolgirls who had been kidnapped by the Islamist group Boko Haram in the Nigerian town of Chibok have been reunited with their families.

In an emotional ceremony in the capital Abuja, one of the girls said they had survived for 40 days without food and narrowly escaped death at least once.

It is unclear how the release was negotiated, but an official says talks are under way to free some more girls.

Of the 276 students kidnapped in April 2014, 197 are still missing.

One of the girls freed said during a Christian ceremony in Abuja: "I was... [in] the woods when the plane dropped a bomb near me but I wasn't hurt.

"We had no food for one month and 10 days but we did not die. We thank God," she added, speaking in the local Hausa language.

Many of the kidnapped students were Christian but had been forcibly converted to Islam during captivity.

Another girl said: "We never imagined that we would see this day but, with the help of God, we were able to come out of enslavement."

Excited relatives were waiting to be reunited with the girls, who were released last Thursday.

One parent said: "We thank God. I never thought I was going to see my daughter again but here she is... Those who are still out there - may God bring them back to be reunited with their parents."

Nigerian authorities have denied reports that captured Boko Haram fighters were swapped for the girls. But one security official told the BBC that four commanders had been freed.

The AP news agency also reported that a "handsome ransom", in the millions of dollars, was paid by the Swiss government on behalf of the Nigerian government.

Nigeria's Information Minister Lai Mohammed said Thursday's release was "the first step" for the liberation of all the remaining girls.

"Already we are on phase two and we are already in discussions," he told journalists on Sunday.

"But of course you know these are very delicate negotiations, there are some promises we made also about the confidentiality of the entire exercise and we intend to keep them."

Photo: Nigeria's Vice President Yemi Osinbajo (right) welcomed the released girls

Some of the kidnapped girls managed to escape within hours of their kidnapping, mostly by jumping off lorries and running into nearby bushes.

In total, 219 girls were captured and taken away. But it appears that some of the girls may have died in captivity.

And reports say that, following more than two years in captivity and after being married off to Boko Haram fighters, some of the girls do not want to go home.