By Jon Leyne
BBC News, Jerusalem
Could this be the start of the long-feared Palestinian civil war?
Gunmen from both factions are on the streets of Gaza
The fighting going on across Gaza has reached a new intensity, while ceasefires come and go.
The sound of automatic gunfire is echoing round the deserted streets of Gaza City. A few civilians scurry for cover. Only the hospitals are doing brisk business.
The latest clashes began after Hamas fighters ambushed a Fatah convoy on Thursday. Hamas said the convoy was bringing in weapons from Egypt.
Fatah then attacked, and briefly occupied, the Islamic University in Gaza, a Hamas stronghold. Fatah said Hamas was using it to store weapons, and claimed it captured a number of Iranians supporting Hamas.
So far no evidence has been produced for any of these inflammatory accusations.
Egypt has openly blamed Hamas, for the first time, for breaking the ceasefire. It is hard to see how Hamas can now trust Egypt to continue its mediation efforts.
The fighting is between members of the Islamist Hamas movement, which controls the Palestinian government, and members of the more secular Fatah faction, whose leader is the Palestinian Authority President Mahmoud Abbas.
It is a struggle for power, a struggle, perhaps, for the future direction of the Palestinian government.
But at the moment it seems as much an expression of the pent-up frustration of a people whose plight gets steadily worse.
Egypt, Saudi Arabia and Jordan are all trying to mediate.
A failed ceasefire agreed earlier this week was negotiated jointly by Egypt and Saudi Arabia. The Egyptians maintain a permanent presence in Gaza City. King Abdullah of Saudi Arabia roundly condemned the fighting earlier this week.
But there are suspicions of less helpful outside intervention as well.
The United States has been giving increasing support to the forces loyal to Mr Abbas, though it denies supplying arms. Egypt and Saudi Arabia, the two main mediators, are strong American allies.
So Hamas may feel the game is stacked against them.
Hamas believe it is just another example of how the outside world refuses to accept the Hamas victory in the Palestinian elections a year ago.
Hamas fear Mr Abbas aims to take over the Palestinian government
They fear Mr Abbas is trying to take over the Palestinian government, as a prelude, perhaps, even to a negotiated peace with Israel.
So this has become the latest arena for the wider and bitter struggle between Islamists and more secular Arabs, between those supported by Iran and those more sympathetic to the West.
There is nothing glamorous about the chaos on the streets of Gaza.
Already the outside world seems almost to have lost interest. Many ordinary Israelis are rubbing their hands with glee, though anarchy on their border may not necessarily be something for them to celebrate.
And although officials say a new ceasefire deal has been agreed between the rival factions, it could still get very much worse.