In an unprecedented move, the US has posted up to $23m in rewards to help track down five leaders of armed groups active in West Africa.

The highest reward of $7mn was offered on Monday for the Boko Haram leader Abubakar Shekau, who last week called on those sympathetic in Afghanistan, Pakistan and Iraq to join the fight to create an Islamic-governed state in Nigeria.

The State Department's Rewards for Justice programme also targeted al-Qaeda in the Islamic Maghreb (AQIM), offering its first ever bounties for wanted fighters in West Africa.

Up to $5m was posted for al-Qaeda veteran Mokhtar Belmokhtar, the one-eyed fighter behind the attack on an Algerian gas plant in January in which 37 foreigners, including three Americans, were killed.

A further $5m was offered for Yahya Abou Al-Hammam, senior AQIM leader, reportedly involved in the 2010 murder of an elderly French hostage in Niger.

Malik Abou Abdelkarim, a senior fighter with AQIM, and Oumar Ould Hamaha, spokesperson for Mali's Movement for Oneness and Jihad in West Africa (MUJAO), were also targeted with the programme promising up to $3mn each for information leading to their arrests.

"AQIM has been increasingly active in North and West Africa. They're one of the pre-eminent kidnap for ransom groups in the terrorist world right now," a senior State Department official told AFP news agency, asking not to be named.

"They cause us a great deal of concern. Anything that we can do naturally to cut down on the capabilities of AQIM, anything that we can do to get information on these people so that we can get them in front of a court ... That is our goal."

'Lawless' Sahel

The US has been increasingly worried about the spread of ultra-conservative groups in Mali and across the vast and lawless Sahel since a military coup toppled the government in Bamako.

Former colonial power France has led a military offensive since January against the opposition fighters in Mali's northern desert, as the west African nation prepares for presidential elections on July 28.

There are fears, however, that the spread of such groups risks destabilising the entire West African region.

Belmokhtar, who was a senior commander for AQIM, broke away from the group last year to set up his own group dubbed the Signatories in Blood.

Branded "The Uncatchable", Belmokhtar also personally supervised the operational plans for the twin car bombings in Niger that killed at least 20 people late last month, according to a spokesperson for his group.

Monday's rewards, which will see a campaign of wanted posters launched in Mali, Nigeria and neighbouring countries, acknowledged the growing links between AQIM and Boko Haram, which is under pressure from a Nigerian army offensive.

"They've had a relationship for some time. They send people back and forth for training, they've done the provision of arms back and forth," the US state department official said.

"The links are ... not quite as solid as some of the other terrorist organisations," he said. "Nonetheless, it's a dangerous link, and it's something that we feel we should try and stop."

Shekau, in a video obtained by the AFP news agency last week, contradicted the military's assertions and claimed his forces had made significant gains against the army while sustaining little damage since the start of the offensive on May 15.

"Under his leadership, Boko Haram's capability has certainly grown," the state department official said.

He highlighted how the group set off "their first improvised explosive device in early June 2011. By August (2011) they used a car bomb against the United Nations facility," an attack which killed 25 people.