Palestinians fired more than 40 rockets from the Gaza Strip into Israel on Wednesday injuring three people, while Israeli air strikes on the territory killed three militants, police and medics said.

Police spokesman Micky Rosenfeld said two of the three injured by the rocket fire were in serious condition, while public radio identified them as immigrant farm workers from Thailand.

The radio added that a house had taken a direct hit by a rocket.

Palestinian medics said earlier that three militants preparing to fire rockets were killed in as many Israeli air strikes on Gaza overnight. Eight others were wounded.

The rocket fire and air raids come amid a spike in tension after an Israeli soldier was seriously injured in a bombing during a routine patrol along the fence in the Kissufim area of Gaza early on Tuesday.

Two of the militants died and seven were wounded in twin air strikes in northern Gaza Strip late on Tuesday, a Palestinian health ministry spokesman said.

A statement from the Islamist movement Hamas, which controls Gaza, said those killed were members of its military wing, the Ezzedine al-Qassam Brigades.

An Israeli army spokesman said the air force had attacked two groups of Palestinians who were about to fire rockets into southern Israel.

A third air strike early Wednesday, on Rafah in the south of Gaza, killed one militant and injured another, a medic said, adding that it had targeted members of the Popular Resistance Committees who were firing rockets into Israel.

The air strike was confirmed by the Israel military.

The raids came after the attack on an Israeli patrol that wounded the soldier was claimed by a leftist group, the Popular Front for the Liberation of Palestine.

Military officials were initially unsure what had caused the explosion, but eventually established it was "an explosive device."

Israeli Prime Minister Benjamin Netanyahu once again warned he would strike back at Gaza militants, which he said were supported by Tehran.

"Today we engaged in exchanges against terrorist aggression that comes from our southern border in Gaza, but it actually comes from Iran and a whole terror network that is supporting these attacks," said Netanyahu.

"The way to fight terror is to fight terror, and that we shall do with great force."

Defence Minister Ehud Barak noted the explosive device was rigged to a gate on the fence, and said Israel takes the attack "very seriously."

The latest incidents came as the emir of Qatar, Sheikh Hamad bin Khalifa Al-Thani, made a landmark visit to Gaza where he inaugurated a multi-million dollar project to rebuild the impoverished Palestinian territory.