Six Nigerien soldiers protecting an oil pipeline to neighbouring Benin were killed during an attack by "armed bandits" in the south of the country, Niger's army said on Sunday.

"A patrol dedicated to securing the pipeline was attacked by armed bandits on the outskirts of the village of Salkam... unfortunately we deplore the deaths of six of our comrades," the army said in its latest bulletin.

A local source told AFP the attack happened on Wednesday.

It was the first such attack against the 2,000-kilometre (1,240 miles) pipeline that links the northeastern oil field of Agadem to the Beninese port of Seme-Kpodji.

The army said it had forced the "attackers to beat a retreat" and take an "unknown" number of dead and wounded with them.

The soldiers targeted were part of an anti-terrorist operation that for years has been patrolling the south of the country close to the borders with Nigeria and Benin.

News of the attack came just two days after the deepening of a diplomatic spat between Niger and Benin.

Earlier this week, a Benin court jailed three Nigeriens, including the deputy general director of the oil company Wapco-Niger, for entering the Seme-Kpodji port illegally.

Niger also shut off the taps to the pipeline despite Benin last week lifting a blockade on Nigerien oil, while the two countries have been embroiled in a spat over the closure of their shared border.

Ties have been strained since last year's coup in Niger.

The oil is essential to the economies of both countries, and to the Chinese company Wapco, which operates the pipeline.

The Niger region bordering Benin has seen several "terrorist attacks" in recent months, according to Niamey's defence ministry.

In early June, the army announced it was creating a "protection force" to tackle "terrorist" attacks at strategic sites, such as uranium mines in the north and the oil field of Agadem, in particular the pipeline to Benin.

The country is also battling jihadists linked to Al-Qaeda, Islamic State and Boko Haram in the west and southeast.