The Palestinian territories are experiencing some of their tensest moments for decades, as inter-factional rivalries spill out into the most serious street fighting yet. The BBC News website's Martin Asser explains why the pressures have reached such a dangerous point.
What has led to the sharp rise in tension?
Rival Palestinian factions Fatah and Hamas have been trying to agree a unity government that would solve a crisis sparked by Hamas's victory in January elections and an international boycott that followed it.
Talks have been difficult and recently hit an apparently irreparable deadlock. Palestinian leader Mahmoud Abbas, of Fatah, has now called fresh elections.
For many months now, law and order has been deteriorating in the Palestinian territories, which are also in the grip of an economic crisis, exacerbated by Israel's military operations and the international boycott.
With no salvation in sight, violence took a nasty turn in Gaza in the middle of December, with the killing of three sons of a Fatah security chief and an apparent attempt on the life of Prime Minister Ismail Haniya, of Hamas.
Amid specific accusations of responsibility, both sides have distanced themselves from attacks on their opponents.
But inter-factional fighting is spilling into the streets of Gaza and the West Bank, and threatens to spiral out of control.
What are the two parties' positions?
Fatah spearheaded the so-called Oslo peace process with Israel - talks which ultmately failed. Fatah leaders remain convinced that ending anti-Israel attacks is the key to forcing Israel into negotiations, leading to independent Palestinian statehood.
Hamas refuses to recognise Israel's legitimacy or give up the armed struggle to liberate the whole of pre-1948 Palestine - land which they regard as lost when the state of Israel was established.
The failure of the peace process, and harsh conditions caused by the occupation, handed Hamas victory in parliamentary elections in early 2006.
Most Palestinian voters had lost confidence in Fatah, which had come to be seen as corrupt and incompetent. Loyalist were bitter about losing power for the first time since the emergence of the party in the 1960s.
Why have unity talks failed?
Fatah's and Hamas's world-views are fundamentally at odds.
At the heart of Fatah's philosophy (and the reason for its international acceptance) is its recognition of Israel's right to exist.
At the heart of Hamas's philosophy (and reason for its isolation by the West) is its unwillingness to give up the armed struggle against an entity (Israel) whose legitimacy it does not recognise and which it accuses of riding roughshod over Palestinians rights.
It has become increasing clear that there is no way to fudge these differences - coupled with which, any compromise in which Hamas retains a guiding hand is almost certain to be rejected by Israel and its backers like the US.
What could happen now?
Mr Abbas has called for fresh elections but Hamas says amounts to a coup against the elected government of the Palestinian territories.
And it is still not clear whether it would even be possible to hold them, given the soaring tensions.
Until now, Palestinians have always been able to back away from the brink, when internecine violence threatened to turn their crisis-ridden lives into the unmitigated disaster of civil war.
This is partly because important bonds of family and clan often cross factional lines in Palestinian society. Any family might have a son in Hamas, another in Fatah, and a third in the security forces.
But the pressure may be building up too much for such valves to continue operating.