GAZA, April 9 (Nidal al-Mughrabi/Reuters) - Israel killed four Palestinian armed men and wounded half a dozen others as it kept up air raids in the Gaza Strip for a third day, responding to increased rocket fire, Palestinian medics said.

Gaza militants continued pounding Israel's south with rockets, wounding five Israelis in the south of the country at around daybreak, Israeli media reports said.

Cross-border violence has surged since Hamas activists fired an anti-tank rocket at a yellow Israeli school bus on Thursday, wounding two Israelis including a teenager listed in a serious condition.

Israel has said it wants to teach Hamas a lesson for that attack which Prime Minister Benjamin Netanyahu said on Friday amounted to crossing a line adding that whoever tries to attack and murder children puts his life on the line.

At least 18 Palestinian armed men and civilians, including an 11-year-old boy, have been killed in retaliatory strikes since Thursday, and 36 since the latest round of bloodletting began in earnest on March 20.

Armed activists have fired dozens of rockets and mortars at Israel, wounding about half a dozen people. Fifty rockets were fired on Friday, and an Iron Dome missile defense system in place since last week intercepted some of them.

A predawn raid Israel killed a local Hamas commander in the southern town of Rafah bordering on Egypt, as well as two of his bodyguards, in a targeted strike on a vehicle, medics said.
              

THREE KILLED INSTANTLY

All three were killed instantly, they said. Five armed men were wounded in a second air raid in northern Gaza, and at daybreak another air strike killed yet another armed man and wounded another.

Israeli air strikes killed nine Palestinian armed men and civilians on Friday, including an 11-year-old boy and an adult found dismembered by a cemetery in a densely populated town east of Gaza City.

An Israeli military spokesman said Palestinians firing rockets at Israel were targeted in that strike.

Among those killed in Gaza on Friday were an elderly Palestinian and two women killed when their house in Khan Younis was hit. Three other women were wounded, hospital sources said.

The Israeli military said uninvolved civilians have apparently been injured in one strike on Friday.

The IDF regrets that the Hamas terrorist organization chooses to operate from within its civilian population, using it as a 'human shield', the statement said.

An Israeli military spokeswoman confirmed the earlier strike, saying the warplanes had targeted Palestinian militants.

Israeli media said a house was damaged and one soldier was slightly injured by rockets and mortars fired on Friday.

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.

(Writing by Allyn Fisher-Ilan; editing by Philippa Fletcher)