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The trip by Qais al-Khazali, the founder and leader of the Asaib Ahl al-Haq militia, was organised by Lebanon's Hizballah, a powerful Lebanese armed movement and arch-foe of Israel.

A video of the visit began circulating on social media on Friday, showing Khazali wearing military uniform during a tour of parts of southern Lebanon.

"We declare our full readiness to stand united with the Lebanese people and the Palestinian cause in the face of the Israeli occupation," he can be heard saying in the recording.

Asaib Ahl al-Haq spokesman Jawad al-Tlibawi told AFP that the visit was "in solidarity with the cause of Arabs and Muslims, with occupied Palestine."

"It's  a clear message to the Israeli entity, as well as solidarity with the Lebanese people if the Israeli entity attacks them," he added.

But in a statement, Prime Minister Saad Hariri said the visit, which took place six days ago, was a "violation of Lebanese laws", without specifying further.

He added that he had instructed authorities to investigate and "take measures to prevent any person from carrying out military activities on Lebanese territory... and to prevent the person in the video from entering Lebanon".

Asaib Ahl al-Haq is an Iran-backed group that is one of the main components of Iraq's Hashed al-Shaabi paramilitary force, which has been pivotal in the fight against the Islamic State group in Iraq.

Hizballah has sent advisers to assist the force in their battle against IS.

The Lebanese group is also fighting in neighbouring Syria, and has been accused of assisting Houthi rebels in Yemen.

Its regional interventions have been a source of tension in Lebanon, and were cited by Hariri when he announced that he was resigning last month.

But he later withdrew his resignation after talks that saw Lebanon's government issue a statement reasserting a policy of non-interference in regional conflicts.

Speculation had swirled around the detention of Hariri, who is a Saudi citizen and grew up in the kingdom, after spending two weeks holed up in Riyad after he resigned. 

Hariri's resignation was widely viewed as being forced by Saudi Arabia, Hariri's backer, bringing its feud with the Iran-backed Hizballah to the tiny country.

Hariri heads a coalition government that includes ministers from the Lebanese militant Hizballah group.

Israel fought a devastating war against Hizballah in Lebanon in 2006 that killed more than 1,200 Lebanese, mostly civilians, and 120 Israelis, most of them soldiers.

Israel withdrew its forces from southern Lebanon in 2000, ending a 22-year occupation, but the two countries remain technically at war and there have been occasional skirmishes on the border.

Agencies contributed to this report