NIGER, Nigeria — Unidentified gunmen kidnapped six people, including four aid workers from Niger and Chad, from the town of Dakoro in central Niger overnight, the aid workers' employers said on Monday.

Nigerien security sources said troops had been deployed on the ground to look for them and aircraft were monitoring traffic into the Air Mountains, where they said the hostages might be taken.

Gunmen linked to al-Qaida factions operating in the Sahel and Sahara zone have kidnapped people in Niger and taken them to neighbouring Mali in the past, although usually they target Westerners for ransom payments.

Those kidnapped included three local staff working for Nigerien medical charity BEFEN and a Chadian working for Alert Sante, the aid groups said in a joint statement sent to Reuters.

Both groups had been treating malaria and looking after malnourished children in the area, they said.

The Islamist takeover of the north of neighboring Mali has created a security void, opening up a safe haven for extremists and organized crime.