A helicopter gunship fired two missiles at a vehicle east of the southern city of Khan Yunis, killing a 17-year-old girl and a 33-year-old man, Palestinian medical and security sources said.
The army confirmed the raid, which wounded another 15 people, saying it was aimed at a car carrying a group of Ezzedin al-Qassam Brigade militants on their way to fire mortars and homemade rockets at Israeli territory.
The move provoked threats of revenge from Hamas' armed wing.
"Faced by this aggression ... we cannot stay with our arms folded. We will hence answer, God willing, the crimes of the occupation," a statement said.
And both Hamas and Islamic Jihad warned the attack could jeopardize the announcement of a ceasefire, with Hamas accusing Israel of deliberately trying to sabotage truce efforts.
"This cowardly assassination confirms that the enemy does not want stability or quiet and hence puts real obstacles (in front of) our continuing inter-Palestinian dialogue and efforts towards a truce," senior official Ismail Haniya told AFP.
The strike came hours after two other militants from the Ezzedin al-Qassam Brigades were killed in a firefight with Israeli troops in the northern Gaza Strip area of Beit Hanun.
Exactly 1,000 days after the Palestinian intifada, or uprising, claimed its first victim, Wednesday's deaths brought to 3,364 the number of people, including 2,536 Palestinians and 768 Israelis.
With the killings raising the spectre of any truce being still-born, a member of Palestinian leader Yasser Arafat (news - web sites)'s mainstream Fatah (news - web sites) faction had said earlier that his group had drafted a truce deal during meetings in Syria with the exiled Hamas and Islamic Jihad leaderships.
He said the draft called for a three-month truce in Israel and the occupied territories and for refraining from obstructing the political process, despite opposition to the international community's "roadmap" for peace.
He also said the three main factions had agreed to condition the truce on the release of Palestinian prisoners.
Speaking on CNN shortly after the airstrike, Hamas official Ismail Abu Shanab dismissed media reports that a ceasefire deal had already been inked.
"The three movements are discussing this issue seriously and, until this moment, nothing official has been declared," he said, saying talk of a ceasefire was premature.
"I think we need some more time to reach an agreement, so let's wait until the official statement is declared."
The Palestinian daily Al-Ayyam had quoted Palestinian sources as saying the factions had agreed that the situation was ripe for a truce and that it could be announced in Cairo by the end of the week.
With the agreement apparently all but finalized, the truce proposal still had to be examined by Arafat.
Palestinian officials explained that the agreement had already been delayed by the killing last week of Hamas military leader Abdullah Qawasmeh.
Hopes of an imminent breakthrough in the conflict were also dampened when a new round of talks between Israeli and Palestinian security officials was postponed.
The two sides have been discussing a partial army withdrawal from designated reoccupied areas, where the Palestinian security services would take over policing duties and prevent anti-Israeli attacks.
The truce and the security deal were expected to coincide with a visit by US National Security Adviser Condoleezza Rice (news - web sites), who is due in the region on Saturday to give the latest US show of support for the roadmap.
In Washington, US President George W. Bush (news - web sites) reacted skeptically to reports on a truce, saying "I'll believe it when I see it."
Speaking at a press conference with European leaders after the annual US-EU summit, he insisted that "the true test" for Middle East peace would be whether extremist groups were dismantled.
He urged European leaders to take "swift and decisive action" against extremist groups like Hamas by cutting off their sources of funding and support.
"Progress toward this goal will only be possible if all sides do all in their power to defeat the determined enemies of peace, such as Hamas and other terrorist groups," he said.
The US-backed blueprint calls for an end to the violence and the creation of a Palestinian state by 2005, but has had little impact on the ground two months after its publication.
Israeli troops continued their sweep for wanted militants in the West Bank on Wednesday, rounding up all the men in a village east of Jenin and arresting some 20 suspected activists in the town of Tulkarem, Palestinian security sources said.
