Mr Fowler became the UN special envoy to Niger in July
A Tuareg rebel group leader in Niger has denied being behind the kidnapping of UN special envoy, Robert Fowler.
Mohamed Aoutchiki Kriska said the rebels were not involved, despite an earlier statement on the group's website saying they had captured him.
Mr Fowler, a Canadian diplomat, was with another Canadian and a local driver when he went missing on Monday about 40km (25 miles) from the capital.
Tuareg rebels have been engaged in armed struggles for several decades.
UN spokeswoman in Niger Hadeta Lonjai said there was full co-operation between the Niger government and the UN in the search for Mr Fowler.
Earlier in a message posted on its website, the Front des Forces de Redressement (FFR) said it had taken Mr Fowler and three other people.
But in a later statement, FFR President Mr Kriska said no hostage taking should be attributed to the FFR, "which itself has fought against this practice that belongs to a past age".
"We hope that Mr Fowler will be quickly returned to the Canadian consulate authorities or to the UN representative because the Nigerien army could endanger his security in order to frustrate his mission and blame rebel movements," he said.
Search under way
The diplomat's vehicle was found on Sunday evening with its engine still running, a UN spokesman said.
UN officials said Mr Fowler, who is based in Canada, was in Niger for meetings with officials.
But Niger's foreign ministry said in a statement that Mr Fowler was in the country on private business and had left Niamey on Sunday morning without informing the authorities.
Security forces were searching for Mr Fowler and his companions, the foreign ministry said.
The group visited the gold mining region of Samira before their disappearance, the ministry added.
The region is some distance from fighting in the north between the army and Tuareg rebels.
Mr Fowler retired from the Canadian foreign service two years after a long career that included postings as ambassador to Italy and the United Nations.
UN Secretary General Ban Ki-moon appointed Mr Fowler his special envoy to Niger in July.
The Tuaregs have traditionally been a nomadic people roaming across the Sahara Desert but some took up arms, saying the Niger government is not doing enough to improve their lives.
The FFR broke away from the better known Tuareg MNJ rebels, who are fighting for greater autonomy and a larger share of northern Niger's vast mineral wealth.
The MNJ has had frequent clashes with the country's army and has also kidnapped foreigners working in the uranium mines.
BBC West Africa correspondent Will Ross says the north of Niger has been dangerous for over a year.
He says the fear is that the rebels are spreading their activities to other parts of the country.