A video posted on YouTube and circulated on Thursday shows two men firing rocket-propelled grenades (RPGs) at a container ship that was attacked on Saturday in the Suez Canal.
Reuters could not verify the authenticity of the video and there were no claims of responsibility for the attack on Islamist websites.
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The video carries a logo featuring a black Islamist flag and the name of the "al-Firqan Brigade". It shows two men, dressed in civilian clothes, firing RPGs into the side of the ship, where they explode.
The ship carries the name "COSCO" on its side and appears to match pictures of the reported target of the attack, the Panamanian-registered Cosco Asia.
An army source said on Thursday that security officials were "sure that the operation was a random operation that is not backed by an organization".
The incident had minimal effect on the ship or on canal traffic. Any major assault on the canal would hurt Egypt's economy, which depends heavily on revenue from the 192-km (120-mile) waterway, the quickest sea route between Asia and Europe.
Security sources say the Canal Authority has received threats from unidentified groups threatening to attack the waterway.
On Thursday, Egyptian Interior Minister Mohamed Ibrahim survived an assassination attempt unharmed when a car bomb battered his convoy and gunmen strafed his vehicle.
He said afterwards that a wave of terrorism by opponents of the military-installed government was just beginning.
After the canal attack, the army said three people had been arrested for firing machine guns at the ship. The head of the Canal Authority, Mohab Memish, later said an RPG had been used.
Militant attacks on security forces in the nearby lawless North Sinai region have been on the rise since the army ousted Egypt's first freely elected president, Islamist Mohamed Morsi, on July 3 after weeks of mass protests against his rule.
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