Gaza - Five Palestinians were killed in an Israeli military operation in Gaza, in the most serious incident since a June truce between Israel and armed groups, Palestinian medical sources said Wednesday.

Four Palestinians were killed in two Israeli air raids near the towns of Dir el Balah and El Bureij, the sources said. An Israeli military spokesman confirmed the raids, adding that they targeted armed groups firing mortars.

Another Palestinian fighter was killed and three other militants were wounded in a shootout with an Israeli army unit in the Gaza Strip near the border with Israel on Tuesday, Palestinian emergency services said.

The fifth casualty was a member of the armed wing of the Islamist Hamas movement in power in the Gaza Strip, the sources said, adding that the incident took place at El Bureij in the central Gaza Strip.

A military spokesman said the Israeli unit was operating close to the border "after the discovery of a tunnel aimed at abducting soldiers."

"The tunnel was discovered 250 metres from the border and was ready to be used," the spokesman added.

He said the Israeli operation was not in violation of a June ceasefire agreement between Israel and armed Palestinian groups.

Separately, unidentified Palestinians fired mortars at Israeli territory, but without causing casualties or damage, a military official said.

At least 544 people -- nearly all of them Palestinians -- have been killed since Israeli-Palestinian peace talks were revived at an international conference in Annapolis, Maryland, in November 2007, according to an AFP tally.