Egypt's Muslim Brotherhood has accused the Israeli spy agency Mossad of being behind Sunday's attack on an Egyptian checkpoint on the border with Israel, BBC News said.

Israel's foreign ministry has dismissed the allegation as "nonsense".

The attack left 16 Egyptian security officers dead. Israeli forces say they later killed at least seven gunmen who broke through into Israel.

Israeli and Egyptian officials have blamed the attack on Islamist militants.

A statement on the Muslim Brotherhood's website said the incident "can be attributed to the Mossad".

It said Israel had carried out the attack in an attempt to undermine the government of Egypt's Islamist President, Mohammed Mursi.

The allegation was echoed by Hamas, the Palestinian offshoot of the Muslim Brotherhood that governs Gaza.

Israel's foreign ministry spokesman, Yigal Palmor, denied the accusation, saying: "Even the person who says this when he looks at himself in the mirror does not believe the nonsense he is uttering".

Earlier on Monday, Egypt's state news agency quoted a top security official as saying that the attackers were "jihadists" who had infiltrated from the Gaza Strip.

A statement by the Egyptian military described them as "enemies of the nation who must be dealt with by force". The country has declared three days of national mourning for the dead officers.

Patrols have been stepped up in the Northern Sinai, and Egypt's Rafah border crossing to Gaza has been indefinitely closed as security forces hunt the remaining attackers.

The raid comes amid growing fears that jihadists have gained a foothold in the thinly populated area.