An Israeli soldier died Friday and four were injured in an accidental explosion during an exchange of gunshots and mortar fire with Palestinians along the Gaza border, an army spokesman said.

A unit came across three so called "terrorists" apparently trying to place explosive devices near the security barrier that separates Gaza from Israel, he said.

They opened fire but for an "unknown reason" a mortar they tried to fire exploded "accidentally" killing one soldier and injuring four others, including an officer, the spokesman said.

Initial reports from the Israeli media had suggested that four soldiers had been wounded by mortar fire and fire from automatic weapons wielded by the Palestinians from the Gaza Strip.

They reported the incident happened near the Berri kibbutz, which lies near the Gaza Strip.

Witnesses inside Gaza told AFP the clash between troops and Palestinian resistance fighters took place east of the town of Khan Yunis in the Palestinian enclave.

They said the firefight lasted 20 minutes, and that one Palestinian was said to have been wounded.

Witnesses also reported that the Israeli forces fired tank shells and a rocket during the incident.

Channel 10 also reported that, following the firefight, significant Israeli reinforcements, including tanks, and backed up by air cover, were deployed along the border.

This was the most serious incident for the army in Gaza since two soldiers were killed in March 2010.

Earlier on Friday, a projectile fired from the Gaza Strip fell harmlessly on southern Israel, Israeli military sources said.

Israel's military usually replies to such attacks with air raids on Gaza, which is governed by Palestinian Islamist movement Hamas.

Late on Thursday, warplanes bombed three sites across Gaza, killing no one, Palestinian security officials and witnesses said.

The air raids hit targets in the southern city of Rafah, in the northern town of Jabaliya and also struck a site which had been previously been used for launching rockets in the Zeitun district, just east of Gaza City, they said.

The Israeli military confirmed attacking only two targets.

"One was a terror site in northern Gaza, the other a terrorist tunnel in the south," a spokeswoman told AFP.

Tunnels from Egypt into neighbouring Gaza are frequent targets of Israeli air raids.

On Wednesday, Palestinian resistance combatants fired seven projectiles, most of them mortar shells, without causing any casualties or damage, the army said.

And that night, Israeli troops shot dead two Palestinians claiming they were trying to get across the border fence into Israel.

In December 2008, Israel launched its devastating "Operation Cast Lead" offensive against Gaza in an attempt to wipe out the resistance.

The 22-day war, which ended in a ceasefire on 18 January 2009, killed 1,400 Palestinians, mostly civilians, and 13 Israelis.