At least six Israelis have died and several others have been wounded in an attack by Palestinian militants at a crossing point between Israel and Gaza.
The usually busy crossing was said to be quiet at the time of the attack
A truck was rammed through a wall and three militants charged through the gap, the Israeli army says.
Israeli security guards returned fire and the three militants were killed.
Joint responsibility for the raid was claimed by three Palestinian groups, including one linked to the Fatah movement of President Mahmoud Abbas.
It is the first big attack since Mr Abbas was elected on Sunday and could be his first major test as leader.
Israel has called on Mr Abbas to go after those responsible for the attack.
Blow for Abbas
Mr Abbas has said previously he would not take on Palestinian armed groups by force, and instead wanted to persuade them to agree to a mutual ceasefire with Israel.
Following the attack, Israeli Transport Minister Meir Sheetrit said the crossing, which is the main commercial entry point for the Gaza Strip and vital for channelling supplies to the 1.3 million Palestinians who live there, would be closed until further notice.
Moderate head of main political faction Fatah
Seen as someone Israel will talk to
Willing to talk peace with Israel
Wants end to Palestinian armed uprising
Pledges to stick to key positions of late Yasser Arafat
The BBC's Alan Johnston in Gaza says the involvement of Fatah-linked al-Aqsa Martyrs' Brigades in the latest assault will be a blow to Mr Abbas.
It shows that even some of the militants in his own political camp do not agree that it is time to halt the violence, our correspondent adds.
Shortly after the incident, an Israeli helicopter fired two missiles at a building in Deir el-Balah in central Gaza, said to belong to the Islamic Jihad militant group.
The Israeli army says in Thursday's attack, the truck was rammed through a wall at the crossing and then three Palestinian militants ran through, firing their weapons and hurling explosives.
Israeli security guards returned fire and the militants were killed.
The heavy exchanges prevented ambulances from reaching those wounded in the bomb attack.
Some reports say the Israelis killed were all civilians.
As well as the al-Aqsa Martyrs' Brigades, the Islamic militant group Hamas and the Popular Resistance Committees said they were involved in carrying out the attack.
Karni is the main crossing point for farm produce and other goods
entering and leaving the Gaza Strip.
It was said to be relatively quiet when the attack took place, at around 2300 local time (2100 GMT).
The attack followed Israeli raids in the Gaza Strip earlier on Thursday in which two Palestinians were killed.
One man died as he drove his pregnant neighbour to hospital in Beit Lahiya. The other was killed in an Israeli raid on the Bureij refugee camp.
Israel, which has occupied Gaza since 1967, is planning to withdraw its troops and settlers from the territory this year.
But our correspondent says groups like Hamas are very keen to portray the Israeli withdrawal as a retreat under fire.