The US government appears close to opening a new front in its fight against Islamist militants by planning a new base for surveillance drones in the west African country of Niger.

American forces are already assisting a French offensive in neighbouring Mali that is aimed at recapturing the country's northern desert territory from the hands of Islamist rebels. On Monday the US signed a military agreement with Niger that paves the way legally for US forces to operate on its soil, prompting a series of reports that the Pentagon was keen on opening a new drones base there.

That news appeared to be confirmed by Niger government sources, who said the US ambassador in Niamey, Bisa Williams, had asked Niger's president, Mahamadou Issoufou, for permission to use surveillance drones and had been granted it.

"Niger has given the green light to accepting American surveillance drones on its soil to improve the collection of intelligence on Islamist movements," a Niger government source told Reuters.

In Washington a diplomatic source told the Guardian that the recently signed deal, known as a "status of forces" agreement, was very broad. "There are no constraints to military-to-military co-operation within the agreement," the source said.

The deal with Niger had been under negotiation for some time but had got a sudden burst of urgency after the dramatic events following the French intervention in Mali. Though French-led forces have swept militants from key cities in northern Mali, the conflict has focused diplomatic efforts on the security threat posed by Islamist groups in the vast wilderness of the Sahel and Sahara.

It is believed that the US only currently desires surveillance drones to be deployed to Niger though the agreement could pave the way for more aggressive armed drones in the future. A spokesman for the Pentagon did not return requests for clarification by email and telephone.

The move would be the latest in a gradual expansion of American surveillance drones in Africa, which have so far been operated from Burkina Faso, Ethiopia and Djibouti in the Horn of Africa. It would represent an acknowledgement that the north and west African regions are becoming a key battleground in the fight against Islamist groups. Aside from the conflict in Mali, Nigeria is plagued by violent Islamists extremists in its northern provinces and Algeria recently saw a high-profile and bloody attack on western workers at an oil industry facility.

US military consultant Robert Caruso said that any base in Niger would probably be similar to the one already in Burkina Faso and use a variety of manned and unmanned craft to carry out spying and monitoring missions. Caruso said it would make up for a lack of human intelligence in the region.

"The reason we are having to push so many drones to the Sahel is because we don't have any human intelligence in the region – either through friendly countries providing it, or through espionage," said Caruso. "We should have humans on the ground, but we don't. When it comes to Mali, US policymakers really don't know what's going on."

However, the use of drones, even if just on surveillance missions, is controversial. American drone strikes have been used to deadly effect in countries such as Pakistan, Yemen, Afghanistan and Somalia but have caused outrage among civil liberties groups over the secrecy that surrounds their operation and the high incidence of civilian casualties. The London-based Bureau of Investigative Journalism has monitored American drone strikes all around the world and calculates that in Pakistan alone there have been some 362 strikes since 2004. They are estimated to have killed up to 3,461 suspected militants in the country and as many as 891 civilians.

Due to the US's broad definition of "militant", many experts believe the number of dead civilians may be much higher. But despite the controversy, President Barack Obama has made the use of drones one of the centrepieces of his national security strategy, overseeing a huge increase in their deployment.

If the use of drones in the Sahel did escalate beyond simple surveillance and into active strikes, some experts warn that the US risks a considerable backlash.

"The use of drone strikes in the Sahel would become very problematic," said one senior diplomatic source, who did not want to be named.

"There are already serious issues with identification of targets and reliability with drones elsewhere, even in Pakistan, where the CIA has a network of informants. But in north Africa they have no idea who they are dealing with whatsoever."

The source added that there was a paucity of intelligence as to who exactly US forces would be targeting when it came to local Islamic groups. "The US has no one integrated into these organisations. Intelligence is extremely poor.

"These groups and subgroups and splinter groups are capable of division at a moment's notice. You might know something about one group but you know nothing about the other. It is an extremely unstable situation," the source said.