At least 13 Palestinians were killed on Tuesday - eight in one incident - in the deadliest fighting between Hamas and Fatah since the rivals formed a unity government to end bloodshed threatening to spill into civil war.
For many Palestinians, the violence in Gaza was particularly disturbing, coming on the "Nakba", an annual day of national reflection over shared suffering in the conflict with Israel.
Palestinian Prime Minister Ismail Haniyeh of Hamas said efforts were underway to "put an end to the fighting", but gave no details. A Palestinian official said Egypt and Saudi Arabia were pressing Hamas and President Mahmoud Abbas's Fatah faction to pull back from the brink.
A Fatah spokesman said Hamas gunmen had killed eight members of Abbas's Presidential Guard in a cold blooded attack near Karni Crossing, Gaza's main commercial entry point into Israel.
The Fatah-affiliated guardsmen were en route to help comrades under assault by Hamas at a training base near the crossing when Israeli forces across the frontier opened fire at them, according to the spokesman, Tawfiq Abu Khoussa.
"Some of the vehicles overturned and some of the men were wounded. The forces retreated but they were ambushed by Hamas gunmen, who finished them off," he said.
An unidentified Fatah security man who said he had been wounded in the incident told Palestinian television: "They came and shot the wounded. They left me, believing I was dead."
Hamas's armed wing denied the allegation, blaming the deaths on Israel and accusing Fatah of killing one of its commanders earlier on Tuesday. The Israeli military said it had fired at two gunmen who approached the border fence, hitting one of them. Raising tensions further, Hamas said one of its senior figures was "executed" at a checkpoint manned by Fatah fighters. Fatah had no immediate comment.
ROCKETS HIT ISRAEL: Israeli Defence Minister Amir Peretz said Israel had no intention of intervening in the internal fighting. In a challenge to Israel, Hamas said it fired a round of rockets at the town of Sderot. One house was hit and at least four Israelis were injured.
"Israel will not restrain itself forever... we will chose the time and place to respond," said Miri Eisin, Israeli Prime Minister Ehud Olmert's spokeswoman. Hamas and secular Fatah formed a unity government two months ago in a Saudi-brokered deal that failed to resolve the crucial issue of control over security forces, and left armed groups fighting deadly turf wars. A truce announced late on Monday never took hold, and at least 22 people have been killed in violence since Friday.
Masked gunmen were in control of Gaza's streets, where a pro-Fatah security officer and another man, who was not immediately identified, were killed on Tuesday.