Assailants beheaded four people and injured 20 in three weekend attacks in Mozambique's volatile north, sources said Sunday, days after US gas giant Anadarko's convoys were ambushed in the area.

One person was killed in that assault whose nephew told AFP he had been beheaded.

The second attack occurred late Saturday in the village of Quelimane near the Tanzanian border.

That attack claimed three lives and left 16 injured, a local source told AFP, adding that seven were treated locally while six seriously wounded patients were transferred to the district hospital in Mocimboa da Praia.

All three fatalities were beheaded after being shot dead, the source added. A third attack targeted Mucujo, 200 kilometres north of Pemba, on Saturday, injuring four people, a military source said.

No group has publicly claimed responsibility.

On Thursday a Mozambican worker was killed and six others were wounded when two road convoys operated by Anadarko came under attack in the region.

They were the first such attacks by militants targeting gas operators in the area.

Anadarko is among the international corporations investing billions of dollars to exploit major gas reserves discovered off Mozambique's northeastern coast.

Last month it advertised in the local media for the supply of armoured vehicles for use in its northern Mozambique operations.

Following the attacks on Anadarko, Interior Minister Jaime Basilio Monteiro said special forces would be deployed to the region to combat the insurgents and protect exploration companies.

Hardline Islamists have launched several deadly attacks in the Muslim-majority Cabo Delgado province in the past year, stoking unrest just as Maputo pushes ahead with exploration efforts.

The Islamist fighters - reportedly seeking to impose Sharia law in the Muslim-majority province - have terrorised remote communities, killing about 200 people since October 2017 The Islamists belong to a group originally known as Ahlu Sunnah Wa-Jama - Arabic for "followers of the prophet" -- but commonly referred to by locals and officials as "Al-Shabaab". It has no known link to the notorious Somali group.

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