Bujumbura--Nkurunziza's precise whereabouts outside the country remained uncertain, amid speculation his plane had to turn back on Wednesday or he that he may have travelled to a third country elsewhere in the region.
An AFP correspondent confirmed the airport in the Burundian capital had been shut and appeared to be in the hands of pro-coup forces.
The attempted coup sparked international alarm, with Washington urging Burundians to "lay down arms, end the violence and show restraint".
Those calls were echoed by the European Union which warned it was "essential the situation does not spin out of control".
UN Secretary General Ban Ki-moon also made an urgent appeal for calm, while the Security Council said it would hold an emergency meeting on the situation on Thursday.
In his message announcing the coup, Niyombare signalled he did not want to take power himself, vowing to form a "committee for the restoration of national harmony" and work for "the resumption of the electoral process in a peaceful and fair environment."
Niyombare is a highly respected figure who was sacked from his intelligence post in February after he opposed Nkurunziza's attempt to prolong his 10-year rule.
More than 22 people have been killed and scores wounded since late April, when Burundi's ruling CNDD-FDD party -- which has been accused of intimidating the opposition and arming its own militia -- nominated Nkurunziza to stand for re-election in June 26 polls.
Clashes between protesters and police early Wednesday, before the coup was announced, left two civilians and a police officer dead, according to the Burundian Red Cross. At least 66 people were injured, it added.
More than 50,000 Burundians have fled the violence to neighbouring nations in recent weeks, with the UN preparing for thousands more refugees.
Opposition and rights groups insist that it is unconstitutional for Nkurunziza, who has been in office since 2005, to run for more than two terms.
The president, however, argues that his first presidential term did not count as he was elected by parliament, not directly by the people.
Asked to rule on the issue, Burundi's constitutional court found in his favour but not before one of the judges fled the country, claiming its members were subject to death threats.
The African Union, EU and United States have condemned the third-term bid by Nkurunziza, a former rebel leader from the Hutu majority and born-again Christian, who believes he ascended to the presidency with divine backing.
AFP
