JERUSALEM, Feb 2 (Reuters) - At least two explosive devices have washed up on Israeli beaches in what security officials described on Tuesday as an attempt by Palestinian armed groups in the Gaza Strip to attack Israel.

On Monday, sappers detonated one such device on a Mediterranean beach in the southern city of Ashkelon a few kilometres (miles) north of the Gaza Strip, police said.

Another was discovered further north on a beach in the port city of Ashdod.

These devices are being treated as intended for use in attacks in Israel, police spokesman Micky Rosenfeld said, but he declined to give details of their nature.

A source in Islamic Jihad in the Gaza Strip said the explosives were part of a joint operation with two other armed groups, the Salahudeen Brigades and al-Aqsa Martyrs Brigades, an offshoot of Palestinian President Mahmoud Abbas's Fatah movement.

These seaborne explosive devices were aimed at (Israeli) ships and other targets. The resistance factions are telling the enemy that they are continuing to think up new means to hurt them in defence of the Palestinian people, the source said.

Avraham Ben Shushan, a former commander of the Israeli navy, said the explosives might have been primitive mines for use against Israeli naval vessels that maintain a blockade of the Gaza Strip, territory run by Hamas Islamists.

There have been attempts to attack Israeli ships before. It appears to me that the devices that have landed on the beaches were washed up after failing to detonate at sea, Ben Shushan told Army Radio.

Israeli police sealed off beaches near the Gaza Strip and ordered Israeli fishing vessels in the area to remain in port after the devices were discovered. (Additional reporting by Nidal al-Mughrabi in Gaza, writing by Ori Lewis; Editing by David Stamp)