Lebanon's prime minister on Saturday criticised a visit by an Iraqi Shiite militia leader to Lebanon's ceasefire line with Israel, saying it violated local law.

The trip by Qais al-Khazali, the founder and leader of the Asaib Ahl al-Haq militia, was organised by Lebanon's Hezbollah, a powerful 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.

In a statement, Prime Minister Saad Hariri said the visit took place six days ago and 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.

Hezbollah has sent advisers to assist the force in their battle against the Islamic State group.

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

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

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

Israel fought a devastating war against Hezbollah 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.