Tunisia said Monday that it was negotiating for the release of 172 nationals being held by a Libyan militia as bargaining chips for one of its commanders detained in Tunis.

The foreign ministry's Arab and African affairs chief Touhami Abdouli said the Tunisians had been detained in western Libya by a militia that is part of the Libya Dawn alliance which controls the capital Tripoli and third city Misrata.

Libyan authorities arrested 400 illegal migrants, including several pregnant women, at dawn on Sunday as they prepared to board boats for Europe, officials said.

Most of the migrants were Somalis and Ethiopians and included "pregnant women", said Mohammed Abdelsalam al-Kuwiri, spokesman for a unit in the Tripoli-based government that combats illegal migration.

Shelling of residential areas of Libya's second city Benghazi killed at least eight children this week, the United Nations said, warning that such attacks may amount to war crimes.

The UNSMIL mission to Libya, in a statement posted on its website, did not specify exactly when or where the children were killed nor indicate who may have been behind the deadly attacks.

Armed groups in Libya are abducting and torturing civilians, the United Nations said Friday, warning that such actions are "war crimes".

"Armed groups across Libya are responsible for abductions of civilians, including minors, on account of their actual or perceived origin, opinion, family and political affiliation," the U.N. Support Mission in Libya (UNSMIL) said in a statement.

NATO and the European Union on Thursday said they wanted to tighten cooperation in the face of "new threats" emanating from the assertive policies of Russia in the ex-USSR and violence to the south of the military alliance's borders.

EU foreign policy chief Federica Mogherini, attending a meeting of NATO foreign ministers in Turkey's southern resort of Antalya, said the two organisations were "different in nature" but facing similar challenges.

The International Criminal Court's chief prosecutor said Tuesday that she was ready to investigate crimes committed by the Islamic State in Libya, where the United Nations is pushing for a peace deal. 

Fatou Bensouda told the U.N. Security Council that the court's jurisdiction "extends to such alleged crimes" by jihadist groups aligned with IS.

Libya's internationally recognized government warned countries on Tuesday against sending vessels into its territorial waters without authorization, after it attacked a Turkish-owned ship and killed a seaman at the weekend.

In a statement posted on Facebook, the government said other countries should "not attempt to undermine (Libya's) authority."

The United States and five European nations reaffirmed their commitment to Libyan "territorial integrity and national unity" Monday, as the North African nation works toward a unity government to emerge from chaos.

"We reiterate that Libya's challenges can only be addressed by a government that can effectively oversee and protect Libya's independent institutions," Britain, France, Germany, Italy, Spain and the United States said in a joint statement.

Libya's internationally recognized government attacked a Turkish-owned cargo vessel from ground and air, killing a seaman, after it entered territorial waters "without permission", a lawmaker said Monday.

"The air force called on it several times to turn back but it failed to comply," said the member of the recognized parliament based in the eastern port city of Tobruk.

Migrants in Libya face "cruelty" and abuse, driving many people to risk their lives in dangerous Mediterranean crossings aimed at reaching sanctuary in Europe, Amnesty International said on Monday.

For years Libya has been a stepping stone for Africans seeking a better life in Europe. Syrian refugees fleeing the conflict at home are also making their way to Libya to try to reach the West.