Gaza- Israeli troops killed four Palestinians in a fire- fight in the southern Gaza Strip Wednesday, further battering an already shaky five-month-old truce.

Palestinian witnesses said the four were killed in a series of airstrikes as the Israelis during the fighting, and Hamas said they had belonged to its military wing.

The Israeli military said in a communique that troops stationed on the border fence between the Strip and Israel had identified several gunmen attempting to place explosive devices.

The troops and the militants opened fire on each other and several mortar shells were launched in the direction of the Israelis. The troops reported hitting four Palestinians in the gunfight, the communique said.

The Palestinian witnesses, however, and police officials said the Israelis had entered the Strip, this sparking the clash with militants positioned near the border fence.

They said Israeli tank shells destroyed a Palestinian house after the army evacuated the residents, according to witnesses.

The ceasefire which came into effect on June 19 was severely tested last week when Israel destroyed a tunnel bring dug under the border, in order, it said, to facilitate the kidnapping of Israeli soldiers.

Five militants and a civilian woman died in the Israeli action, which prompted the militants to resume rocket barrages on southern Tel Aviv after months in which single missiles had only been fired sporadically.

The latest gunfight comes a day after Israeli Prime Minister Ehud Olmert said said an armed clash between Israel and Hamas was "just a question of when, and not of if."

The Egyptian-mediated truce of June 19, between Israel and the Gaza militias, came in an attempt to calm the Gaza area, Palestinian rocket fire on southern Israel, and Israeli air and ground strikes on rocket-launching militants, were a regular, almost daily, feature.

Under the truce, the militants agreed to cease the missile launching, and Israel said it would widen the scale of humanitarian aid it allowed into the enclave.

Israel has shut off the Gaza Strip since June 2007, when Hamas militants there routed forces loyal to the more moderate, secular Fatah party of President Mahmoud Abbas and seized control of security bases in the salient. (dpa)