CAIRO: Six supporters of Egypt's outlawed Muslim Brotherhood were killed in clashes with police near the Giza pyramids in Cairo on Friday, security sources said, as Egyptians marked the Muslim holiday of Eid al-Fitr.

Security forces intervened and arrested 15 armed Brotherhood members, the state news agency MENA quoted an Interior Ministry source as saying. Twenty Brotherhood members were arrested in Egypt's second city Alexandria, MENA said.

The violence erupted in several different locations in the Giza area, the sources said. A Health ministry official confirmed the deaths.

The Egyptian army toppled President Mohamed Mursi of the Muslim Brotherhood in 2013 after mass protests against his rule.

Security forces then cracked down on Brotherhood supporters, killing hundreds in street protests and arresting thousands of others. Top Brotherhood leaders were sentenced to death in mass trials.

The measures, widely criticised by human rights groups, weakened Egypt's oldest Islamist movement, which had said it would return to power through street protests. Instead, demonstrations have largely faded.

The Brotherhood, declared a terrorist group by authorities, says it is committed to peaceful activism.

Islamist lawyers close to the Brotherhood and Western diplomats say young members frustrated by imprisoned old guard leaders and what they call widespread repression are resorting to small-scale violence like homemade bombs.

While President Abdel Fattah al-Sisi has neutralised the movement, a militant insurgent group affiliated with Islamic State jihadists are seen as a major security threat from their strongholds in North Sinai.

Recently renamed Sinai Province, the group said on Thursday it fired a rocket at an Egyptian naval vessel in the Mediterranean Sea near the coast of Israel and the Gaza Strip.

The Egyptian military said in a statement that a coastguard launch had exchanged shots with "terrorist elements", causing the vessel to catch fire. It said there was no loss of life.

Such incidents at sea are rare, though Egypt is battling an increasingly brazen Islamist insurgency in the Sinai that lies between Israel, the Gaza Strip and the Suez Canal.

Aside from insecurity in the Sinai, officials are also concerned by militants thriving in the chaos of Libya, which shares a long porous border with Egypt.