CAIRO (REUTERS) - Five members of Egypt's security forces were killed in an attack by suspected Islamist militants on military checkpoints in North Sinai on Saturday, security sources said.

The deaths occurred when several mortar rounds hit the checkpoints, said the sources.

North Sinai is the epicentre of an insurgency waged by militants who support Islamic State in Iraq and Syria (ISIS), the ultra-hardline Sunni group which controls large parts of Iraq and Syria and has a presence in Egypt's neighbour Libya.

Islamic State supporters said on a Twitter account that followers of the group had attacked security forces in North Sinai.

Egypt's state news agency Mena said the attack by terrorists - the term the government uses to describe militants - involved rockets and shooting and that clashes were continuing.

President Abdel Fattah al-Sisi says militants are an existential threat to Egypt, other Arab states and the West.

While a tough crackdown has neutralised his enemies in the mainstream Muslim Brotherhood, Islamic State's Egypt affiliate, Sinai Province, remains resilient despite steady pressure from military operations.

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

The group has killed hundreds of soldiers and police since the army toppled President Mohamed Mursi of the Muslim Brotherhood in 2013 after mass protests against his rule.