At least seven civilians, including three children, have been killed in Egyptian air strikes in northeast Libya.

The bombings came as Cairo vowed to find those responsible for the beheadings of 21 Egyptian Coptic Christians kidnapped by fighters pledging allegiance the Islamic State of Iraq and the Levant (ISIL) in Libya's Sirte.

Sources told Al Jazeera on Monday that at least seven people were killed in air strikes in the coastal city of Derna after Egypt's President Abdel Fattah el-Sisi vowed to "punish" those responsible for the beheadings.

Egypt's military said it carried out the raids early on Monday against ISIL camps, training sites and weapons storage areas.

However, photos published on social media, purportedly, showed several damaged residential areas in Derna.

Omar al-Hassi, the head of Libya's legally-installed government in Tripoli, called the Egyptian raids "terrorism" and denounced them as a "sinful aggression".

"This horrible assault and this terrorism that's been conducted by the Egyptian military represents a violation of sovereignty in Libya and is a clear breach of international law and the UN charter," Hassi said.

Following the raids, Sisi deployed the armed forces to protect key installations and buildings in Egypt.

Fighters pledging allegiance to ISIL released a video on Sunday purporting to show the killing the Egyptians kidnapped in Libya.

The Egyptian government and the Coptic Church confirmed the authenticity of the footage, which showed the migrant workers, all wearing orange jump suits, being beheaded near a waterfront said to be located in the Libyan province of Tripoli.

The men were seized in two attacks in December and January from Sirte in eastern Libya.

PHOTO CAPTION

Egyptian President Abdel Fattah al-Sisi

Al-Jazeera