At least 20 people have been killed in a fourth day of gun battles in the Gaza Strip between the rival Palestinian factions, Fatah and Hamas.
Both Hamas and Fatah are said to have called a new ceasefire
Both groups have called a renewed ceasefire to end the violence in which nearly 40 people have died, but gunfire was still being heard after it began.
A BBC correspondent described the fighting as the worst he had ever seen.
Four Israelis were also injured by a rocket attack, prompting their prime minister to order "a severe response".
Shortly after Israeli Prime Minister Ehud Olmert said his country's "policy of restraint" could not continue, an Israeli strike on a Hamas training camp in southern Gaza killed four people.
In a later Israeli strike, a Hamas militant was killed and two other Palestinians wounded in a strike in northern Gaza, Palestinian sources said.
Palestinian militants responded on Wednesday evening with a further barrage on the southern Israeli town, causing a brief blackout with an electricity transformer was hit.
Gun battles continued throughout the day in the Gaza Strip between Fatah, loyal to President Mahmoud Abbas, and Prime Minister Ismail Haniya's Hamas group.
The area around the Palestinian president's compound was besieged by gunmen.
Staff in the building that houses most of Gaza's broadcasters were forced to seek refuge for several hours in one room as Fatah gunmen traded gunfire with Hamas militants on the ground.
Ibrahim Adwan from the BBC's Gaza bureau, who was inside the building, said the siege had now ended and fighting had subsided in that area.
He said the reporters had been stuck on the ninth floor with heavy shooting towards the building.
"Fatah militants were occupying the roof of our building, and the Hamas people were shooting towards the building," he said.
Mr Adwan described the latest infighting as the heaviest he had ever seen.
Palestinian Information Minister Mustafa Barghouti described Wednesday's violence as shameful.
"They keep making agreements, and then they violate it within minutes," he told the BBC.
"When we formed the national unity government, it was formed specifically to prevent internal fighting, and to open the road of protecting democracy."
The UN secretary-general, Ban Ki-moon, repeated his calls for an immediate end to the "unacceptable attacks" on Palestinian Authority installations and personnel, which he said endangered civilians throughout Gaza.
Mr Ban also said the rocket attacks on Israel were "equally unacceptable".
Hamas said it had ordered a unilateral ceasefire from 1700 GMT and President Abbas said Fatah would also stop fighting.
Three earlier truces have been broken and initial reports said clashes were continuing, including around the National Security Headquarters in northern Gaza.
One official also said gunmen had fired upon the guards protecting Mr Haniya's residence, but later said the gunfire did not appear to be hostile.
BBC's Arab affairs analyst Magdi Abdelhadi says the collapse of ceasefires suggests the armed supporters on the ground may no longer be paying attention to orders from their political leadership.
A Palestinian government of national unity had been agreed between the factions in February in an attempt to end the factional violence.
But about 40 people have now been killed and 114 injured since fierce fighting broke out in Gaza on Sunday, according to figures from the UN humanitarian affairs office.
Interior Minister Hani Qawasmi, an independent, resigned on Monday after the violence started.