Israeli air strikes in the Gaza Strip killed three militants from the Islamist group Hamas and three Palestinian civilians on Friday, in a second day of a fresh upsurge in the conflict.
At least 29 Palestinians have been killed since the latest spasm of violence erupted on March 20, with 11 killed over the past two days by Israeli action after an attack on a school bus which injured two Israelis including a teenager.
The Israeli military on Friday said it "identified two terrorist squads from Hamas" and hit the militants from the ground and air. A later strike near the coast killed a third militant and wounded another, Gaza medical sources said.
Hamas said a local commander was also badly injured, and it responded by firing six rockets at Israel from the south.
An elderly Palestinian and two women died earlier when their house in Khan Younis was hit and three other women were wounded, according to hospital sources.
An Israel Defence Forces (IDF) statement said "uninvolved civilians have apparently been injured" in one strike.
"The IDF regrets that the Hamas terrorist organisation chooses to operate from within its civilian population, using it as a 'human shield'," the statement said.
At least 15 rockets have been fired into Israel since dawn, causing damage but no injuries, said a police spokesman. Police were restricting traffic near the border area and there appeared to be no early end in sight to operations.
"We are in the middle of an event," General Tal Russo, head of Israel forces Southern Command, told reporters. "We are considering all actions, and we are in the midst of it."
Hamas had been hit hard, he said, but it was not over yet.
"We are considering everything. We are looking short-term, long-term. There are many tools in the box."
Red line crossed
Two years of low-level skirmishing on the border escalated suddenly last month when the Islamist Hamas group which rules Gaza fired a barrage of rockets at Israel, triggering a surge of fighting which killed 18 Palestinians.
Analysts in Gaza said Hamas wanted to bolster its claim to leadership of the divided Palestinian national movement and divert attention from popular demands -- fuelled by the "Arab Spring" -- for an end to the split with its Fatah rivals.
That spurt of violence subsided, but fighting flared again on Thursday when Hamas gunmen fired an anti-tank missile at an Israeli school bus, wounding two. Israel retaliated with planes and armoured forces, killing five Palestinians.
An Israeli analyst said Hamas was plainly smarting from recent setbacks, including an April 2 Israel airstrike that killed three Gazans, and which it vowed to avenge.
But firing a long-range anti-tank weapon at a clearly marked yellow school bus may have been a further miscalculation.
"The attack yesterday on a children's bus is crossing a line," Prime Minister Benjamin Netanyahu said in Prague.
"The Israeli army responded immediately during the night and will continue to act with determination. Whoever tries to attack and murder children puts his life on the line," he said.
The United Nations and European Union called on the sides to show restraint and bring an end to the latest round of fighting.
Gaza militants on Thursday fired 45 rockets and mortars into Israel. Israeli forces struck targets throughout the densely populated territory, including Hamas positions, smuggling tunnels along the border with Egypt and Gaza's derelict airport.
Hamas has again said it was interested in restoring "calm" to the front and was trying to get smaller militant groups to halt attacks.
During Thursday's barrage, Israel used for the first time its short-range Iron Dome interceptor. Its missiles destroyed in mid-flight a rocket heading for the city of Ashkelon.
Minister of Home Front Defence Matan Vilnai said Israel will consider deploying further Iron Dome systems near Gaza to join the two currently stationed there, but hinted at budgetary restraints.
Hamas, which does not recognise Israel's right to exist, seized control of the Gaza Strip in 2007 from forces loyal to Western-backed Palestinian President Mahmoud Abbas. The two wings of the Palestinian national movement remain enemies.