Mr Fowler became UN special envoy to Niger in July 2008
A UN special envoy and three other foreigners abducted by al-Qaeda in Niger have been freed, the presidency of neighbouring Mali says.
Robert Fowler and his aide Louis Guay, both Canadians, were released by al-Qaeda's North African wing, said presidential spokesman Seydou Cissouma.
Two female tourists abducted in January, a German and a Swiss, were also released, he added.
But a Swiss man and a British man seized with them remain captives.
The captors are believed to be from al-Qaeda in the Islamic Maghreb.
Some reports suggest the six were initially abducted by Tuareg rebels operating along the border between Niger and Mali.
Unnamed sources at the UN confirmed Mr Fowler and Mr Guay had been freed but they said they were awaiting further details, Reuters news agency reports.
There was no immediate comment from the Canadian government.
Asked about the ex-hostages' condition, Mr Sissouma said he believed them to be in good health.
The freed hostages have been taken to the north Malian town of Gao, an unnamed local politician told AFP news agency.
Mr Fowler, his aide and an unnamed driver disappeared in December about 40km (25 miles) from Niger's capital, Niamey. Their car was found abandoned.
In January, the group of tourists was kidnapped along the border with Niger after attending a Tuareg cultural festival in Mali.
A spokesman for al-Qaeda in the Islamic Maghreb announced the group was holding the six foreigners in February.
It emerged in early 2007 from a feared Algerian militant organisation, the Salafist Group for Preaching and Combat, which had aligned itself with Osama Bin Laden.