Palestinian security chief Mohammad Dahlan resigned today after weeks of criticism over the routing of his forces by Hamas Islamists in the Gaza Strip in June, senior Palestinian officials said.
The news came as Israel kept up its raids of militant strongholds, killing at least four militants in a series of air strikes and ground raids in Gaza, and arresting a leader of Islamic Jihad in the West Bank town of Jenin, Palestinian medics and militants said.
One of those killed, Omar al-Khatib, a commander of Islamic Jihad, had earlier survived an Israeli attack on Tuesday by jumping out of his car in Gaza City as an Israeli aircraft flew overhead. Its missile only damaged a building on that occasion.
Mr Dahlan (46) rose through the ranks of President Mahmoud Abbas's secular Fatah movement as a protege of the late Yasser Arafat.
But he has disappointed US sponsors who hoped he could counter Hamas in both Gaza and the Israeli-occupied West Bank.
Abdel-Salam Abu Askar, an aide to Mr Dahlan, said in a statement that he had "tendered his resignation" as Mr Abbas's national security adviser on medical grounds. Mr Dahlan, who is recovering from surgery performed in Germany on both knees, is now having physiotherapy in a hospital in the Balkans, he said.
Jibril Rajoub, a former security chief and longtime rival of Mr Dahlan, is widely seen as a possible successor.
Mr Rajoub has had good relations in the past with US officials, who have stepped up efforts in recent months to train and equip Mr Abbas's forces.
Mr Dahlan had already been sidelined since the fall of Gaza six weeks ago in the fighting with Hamas in which 100 were killed.
Aides to Mr Abbas said the president expected to receive a report on Friday into events in Gaza that is likely to point out Mr Dahlan's responsibility for the failures of security there.
Mr Dahlan said in June he expected to be blamed for Fatah's defeat "because I wasn't there and I'm very close to (Abbas)". He added: "Definitely mistakes were made ... Even myself, I was gone for 50 days," during the surgery.
Mr Abbas aide Azzam al-Ahmed said today: "He was fully responsible for the events in Gaza events ... He feels that he shoulders the responsibility ... Resignation does not exempt him from responsibility."
US support for Fatah-run security forces has been built up this year -- at first despite international sanctions against the Hamas-led government and now, since Mr Abbas ejected Hamas from government following the rift with Gaza, as part of a wider strategy to bolster Abbas as a bastion against the Islamists.
Mr Dahlan is a fierce opponent to Hamas, and is seen as an obstacle to any reconciliation between the Islamist group and secular Fatah.
Hamas says it acted in Gaza because they believed Mr Dahlan and other Fatah leaders were about to attack them.
Meanwhile the Israeli military confirmed several air strikes today. In one, the target was a Hamas militant aiming a grenade launcher at forces on an incursion to halt rocket firings at the Jewish state. In the other, it said it killed three Islamic Jihad militants.
Militants fired a rocket at Israel during the raid, causing no injury, a military spokeswoman said. The Islamic Jihad group claimed responsibility for the rocket fire.
A fifth Palestinian man was killed in the West Bank when an Israeli soldier struck him with a baton at a military checkpoint as he threatened the unit there with a knife, then scuffled with the troops, a military spokeswoman said.
Violence has been rare in the recent past at the more than 500 checkpoints and roadblocks Israel operates in the West Bank, which restrict Palestinian movement in the territory.
Israel says the measure is necessary to prevent suicide bombers from reaching its cities.