The three fighters from al-Quds Brigades were killed early on Friday just hours after Israeli soldiers evacuated an army base.
Afterwards, the Islamic Jihad said it launched five rockets at Israel from north Gaza to avenge the killings, in the first such attack since Israeli forces completed a pullout from the territory 10 days ago.
No casualties or damage were reported from the rocket which on Friday crashed into a field in southwest Israel near the border with the northern Gaza Strip.
Military sources could confirm only one rocket impact. "Other explosions were heard in the region but no (other impact in Israel) has been confirmed," a source said.
Khalid al-Batsh, a spokesman for Islamic Jihad, held Israeli Prime Minister Ariel Sharon and Defence Minister Saul Mofaz responsible for what he called "a deliberate assassination".
Also on Friday, Palestinian police and Hamas fighters briefly exchanged fire, leaving an armed Hamas man wounded, security officials and hospital doctors said.
The shooting took place near the headquarters of the Palestinian Preventive Security Service in Gaza City.
Islamic Jihad's Al-Batsh told Aljazeera on Friday that the group will not allow Sharon to settle political accounts with former prime minister Benjamin Netanyahu at the expense of Palestinians.
"But we assure him [Sharon] that this would not happen," he said.
Palestinian police at Dotan base
vacated by Israel on Thursday
Al-Batsh added the "crime" was a clear message that the Israeli occupation still exists and Palestinians are still being targeted.
Israeli troops went into the village of Alar in the Tulkarim area after midnight, Palestinians and Israeli military officials said.
The soldiers planned to arrest suspects, the officials said. Two armed Palestinians fled the scene and opened fire on Israeli soldiers, who returned the fire. Palestinians said the two men, both Islamic Jihad fighters, were killed.
A third Islamic Jihad fighter, who had been in hiding, came out towards the troops slightly later, and during another exchange of fire he was also killed, an Israeli army spokeswoman said.
"The operation was a direct assassination of both activists, not an arrest attempt, as the three killed were activists of al-Quds Brigades, Islamic Jihad's military wing," Al-Batsh told Aljazeera.
On 25 August in a similar raid in the nearby Tulkarim refugee camp, Israeli forces killed five Palestinians, at least two of them unarmed teenagers.
Security responsibility for Gaza
now lies with the Palestinians
Despite a ceasefire declared in February that has drastically reduced violence after nearly five years of conflict, Israel continues to target Islamic Jihad cells in the Tulkarim area.
Shortly after the truce was declared, Islamic Jihad claimed responsibility for a bombing attack in Tel Aviv that killed five Israelis.
In Washington on Thursday, US President George Bush asked Jordan's visiting King Abdullah II to meet Israeli Prime Minister Ariel Sharon and Palestinian leader Mahmoud Abbas to help advance Middle East peace negotiations.
Bush said Abdullah "graciously agreed".
On Thursday afternoon, Israeli forces evacuated the Dotan base, last used as a command post during last month's "disengagement" operation that included evacuating four West Bank settlements.
Though not part of the disengagement plan that included removing all 21 Jewish settlements from Gaza, the evacuation of the Dotan base signalled that Israeli forces have "completed their mission" in connection with the withdrawal, the military said.
"The operation was a direct assassination of both activists, not an arrest attempt, as the three killed were activists of al-Quds Brigades"
Islamic Jihad spokesman
Tragedy struck a few hours earlier, when a group of Palestinians entered the base, thinking mistakenly that it was already empty. Israeli soldiers opened fire, killing
19-year-old Allah Khamtouni.
The Israeli military commander said soldiers, fearing a bomb attack, fired warning shots, but one killed the teenager. He was unarmed.