Israeli air strikes killed two Palestinian militants and a civilian in the Gaza Strip on Monday in a fourth day of cross-border hostilities, medical sources said.

The latest attack brought the casualty toll to 21 Palestinians dead, most of them gunmen, and at least 74 wounded, most of them civilians, since the violence flared last Friday.

It follows a familiar pattern in which militants launch rocket attacks and Israel carries out air raids in the Hamas-controlled enclave. But the bloodshed has usually ended after a few days with an informal truce.

Gaza's Hamas leadership, whose own cadres have kept out of the fighting, said on Sunday that neighbouring Egypt was working to stop the violence and consulting with other militants.

A Palestinian official close to the mediation told Reuters that Israel had agreed to a midnight ceasefire.

But Islamic Jihad - an Iranian-backed faction behind most of the rocket fire - had baulked, insisting that any truce include an undertaking by Israel not to target militant leaders in future air strikes.

Two chiefs of the Popular Resistance Committees (PRC) faction, accused by Israel of planning to attack it via Egypt's Sinai desert, were killed in such a strike on Friday. Israel signalled no end to these so-called preventive targeting operations.

If the Qassam (rocket) fire ends, the Israel Defence Forces will not continue (strikes), but the Israel Defence Forces will continue thwarting any attempted terrorist attacks, military spokesman Brigadier-General Yoav Mordechai told Israel Radio.

He confirmed there had been contact with Egyptian mediators.

An Islamic Jihad gunman was killed on Monday while launching a rocket from south Gaza, the group said. Another died and three others were hurt when their motorised rickshaw was hit.

Hospital officials said a 15-year-old schoolboy was killed in a separate air strike on Monday, with the death toll since Friday rising to 18 gunmen and three civilians.

At least 74 Palestinians, most of them civilians, have been wounded over the past three days. More than 30 were hurt on Monday in an air strike on the home of an Islamic Jihad commander in the northern Gaza Strip. Islamic Jihad said he survived assassination but it did not elaborate.

Palestinian attacks against Israel have disrupted normal life in the south of the country and forced many schools to close on Sunday and Monday. Alerts to residents of southern towns to take shelter from incoming rockets punctuated radio programmes.

PAINFUL BLOW

Asked about the slain schoolboy, the Israeli military said it had no information about any air strikes in the area at the time. But it said the air force had struck repeatedly in Gaza overnight in response to more than 25 rocket launches.

Islamic Jihad and the PRC, armed groups largely independent of Hamas, have said they fired most of the scores of rockets launched at Israel since Friday.

Some experts in Gaza believe Hamas had provided some smaller groups with ammunition but avoided direct participation in the current fighting out of concern Israel would step up its attacks in the enclave.

Hamas is also keen to avoid any long-term military campaign as it struggles to adapt to political upheaval in Arab countries such as Egypt, which shares a border with Gaza, and in Syria, where the group has abandoned its traditional headquarters.

Israeli Defence Minister Ehud Barak said in a statement on Sunday it would take several more days until the violence ended and Prime Minister Benjamin Netanyahu said Israel would hit hard at militants who launched rockets at its towns.

Though serious, few in Israel expect the bloodshed to lead to a ground assault reminiscent of the 2008-2009 Gaza war in which some 1,400 Palestinians and 13 Israelis were killed.

Israel is not keen to see an escalation, Israel is not keen to hurt innocents - Israel is absolutely opposed to this, said Interior Minister Eli Yishai told Israel's Army Radio.

For now, it (fighting) is on this kind of scale. But if it will prove protracted, then without a doubt there will be a powerful, painful blow so that this will not continue.

The violence has raised international concern.

France called for an instant return to calm and restraint in order to avoid an escalation which would risk once again harming civilians. Russia also urged an end to the fighting and said the use of any violence from which the civilian population suffers is totally unacceptable.

Gaza, home to 1.7 million people, was under Israeli occupation from 1967 until 2005 and remains under blockade.

Islamist Hamas has controlled Gaza since 2007 and is fighting for an independent Palestinian state but has shunned the stalled peace process supervised by international powers and refuses to recognise the Jewish state.

Islamic Jihad is less influential than Hamas but shares the same ideology, which advocates Israel's ultimate destruction.

(Writing by Dan Williams; Additional reporting by Paris and Moscow bureaux)