IDF strikes killed four Hamas militants and wounded half a dozen others on Saturday as Israel kept up air raids in the Gaza Strip for a third day, responding to increased rocket fire, Palestinian medics said.

An Israeli air strike on a vehicle on Saturday killed a Hamas commander in the southern Gaza Strip and two of his assistants. Several hours later, another militant was killed by an IDF strike.

The Israel Defense Forces spokesman's office confirmed that IAF jets attacked three top Hamas officials in the Gaza strip, as well as a smuggling tunnel and a truck carrying ammunition, after southern Israel suffered a barrage of rockets overnight.

Hamas said 29-year-old Tayser Abu Snima and two of his assistants were killed in the blast.

The IDF also attacked several facilities of Hamas' military wing in the area of Jabalya.

Palestinians sources said the attack raised to 18 the number of Palestinians killed in retaliatory strikes since a rocket fired from the Hamas-ruled territory struck an Israeli school bus on Thursday seriously wounding a teenager.

The early morning attack brought the two-day death toll from Israel's ongoing retaliation to 18 Gazans - 10 militants, a Hamas policeman and six civilians - amounting to the bloodiest tally since Israel and Hamas wrapped up their three-week-long war more than two years ago.

The IDF said it had "identified two terrorist squads from Hamas" and hit them from the ground and air.

Two years of low-level skirmishing on the border escalated suddenly last month when Hamas, which rules Gaza, fired a barrage of rockets at Israel. Hamas had largely withheld fire at Israel since a Gaza war of late 2008 in which 1,400 Palestinians and 13 Israelis were killed.

On Friday, a barrage of rockets and mortar shells was fired at Israeli towns near the Gaza Strip, and one of the shells exploded in a chicken coop in the Eshkol Regional Council in the Negev.

Rockets were fired near the south Israel city of Ashkelon, and the Iron Dome missile defense system was successful at intercepting three of them. One rocket fell in an open area near Ashkelon, and no one was hurt.

Later on Friday, two mortars were fired at a Negev town and two additional mortars exploded in open areas. Although no one was hurt, the mortars exploded near homes and chicken coops, causing damage.

On Thursday, IDF strikes in Gaza resulted in the death of five Palestinians, after Gaza militants fired an anti-tank missile at an Israeli school bus, wounding two people.

Hamas announced Thursday that a unilateral cease-fire on the part of all militant factions in the Gaza Strip was to come into effect at 11 P.M. local time, following a meeting between the factions.

Hamas gunmen on Thursday fired an anti-tank missile at an Israeli school bus, wounding two. Hamas took responsibility for the bus attack, and an Israeli analyst said it was plainly smarting from recent setbacks, including an April 2 Israel airstrike that killed three Gazans, and which it had vowed to avenge.

The United Nations and European Union on Friday voiced concern over the rising tensions and called on the sides to show restraint and bring an end to the latest round of fighting.