By Abdoulaye Massalatchi

NIAMEY (Reuters) - Niger security forces have cornered gunmen who kidnapped six people, including four aid workers from Niger and Chad, from the town of Dakoro in central Niger overnight, military sources said on Monday.

"Defense forces were able to locate the kidnappers moving in the Tassara-Tillia pastoral area (near the border with Mali)," one military source said, requesting not to be named.

"The kidnappers are virtually surrounded. However, the forces have been instructed not to endanger the lives of hostages," the source added.

At least one hostage was injured when the gunmen broke into the house and seized the aid workers as they were sleeping, state radio and local government officials said.

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.

There were no details on the other two hostages.

Nigerien security sources said troops had been deployed on the ground to look for them and aircraft were monitoring traffic into the Air Mountains and other northwestern border areas, where they said the hostages appear to have been taken.

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

"The latest information we have is that they have made it into Agadez region through the area west of Ingall," a security source told Reuters, asking not to be named and referring to towns in Niger's vast desert north.

"Measures have been taken to stop them from entering Mali and Algeria," the source added.

Ransoms paid for Western hostages have earned Islamist groups in the region millions of dollars officials say, even though payments are never officially confirmed.

Aid agencies rely on staff from the region to carry out virtually all their work in Niger, which is recovering from a food crisis this year, due to the threat against Westerners.

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

(Additional reporting David Lewis; Writing by David Lewis and Bate Felix; Editing by Michael Roddy)