The Pentagon has secretly expanded its global network of drone bases to North Africa, deploying unmanned aircraft and US military personnel to a facility in Tunisia to conduct spy missions in neighbouring Libya.

The Air Force Reaper drones began flying out of the Tunisian base in late June and have played a key role in an extended US air offensive against an Isis (Islamic State) stronghold in Libya.

The Obama Administration pressed for access to the Tunisian base as part of a security strategy for the broader Middle East that calls for placing drones and small Special Operations teams at a number of facilities within striking distance of militants who could pose a threat to the West.

US officials, speaking on condition of anonymity to discuss an operation that has not been acknowledged, said the drones being flown out of Tunisia were currently unarmed and were principally being used to collect intelligence on Isis targets in Sirte, Libya, where the United States has conducted more than 300 airstrikes since August.

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US officials said they sought access to the air base in Tunisia to close a critical "blind spot" for US and Western intelligence services in North Africa, which has become Isis' largest base of operations outside Syria and Iraq. The region is also home to al-Qaeda-linked fighters.

Obama Administration officials say they have tried to shore up Tunisia's fledgling democracy and position the country as a key counterterrorism partner in the region. Though the drones operating out of Tunisia currently conduct only surveillance missions, US officials said they could be armed in the future, if Tunisia gives the United States permission. The Tunisian Embassy in Washington declined to comment.

The US military has other drone bases on the African continent, from Niger to Djibouti. But officials said they were too far from populous areas on the Libyan coast to be useful in day-to-day counterterrorism operations there. The longer drones have to travel to reach their destinations, the less time they have to "loiter" over their targets.

For lethal strikes in Libya, the US military has relied on manned US aircraft based in Europe and armed drones flown out of Naval Air Station Sigonella on the Italian island of Sicily.

Sigonella is relatively close to Sirte, but flights from the base are routinely cancelled because of cloud cover over the Mediterranean and other weather-related issues, officials said.

US logistical concerns about using Sigonella and other bases in Europe for operations in North Africa prompted the Pentagon's push for a facility on Tunisian soil.

The Obama Administration has kept its negotiations over access to the base secret for more than a year because of concerns that Tunisia's young democracy, worried about being closely associated with an outside military power, would pull out of the talks, or that militants would step up attacks in the North African country.

Isis has already claimed a number of attacks in Tunisia over the last two years, including the killing of dozens of foreign tourists at a beach resort in 2015.

Defence officials said the Pentagon has deployed about 70 military personnel to Tunisia to oversee drone operations there.

As part of the new arrangements, the Obama Administration agreed to share intelligence from the Reapers with Tunisian security forces to help them improve border security. But so far, the United States has made drone flights inside Libya the priority.