Israeli troops have shot dead at least three members of Hamas' armed wing in a clash on the Gaza-Israel border, Hamas and the Israeli army have said.
Militants ended a truce and launched rockets on Tuesday
The incident took place at about 0830 (0530 GMT), north-west of Gaza City.
The Israeli military said four Hamas members were caught trying to plant a bomb on the security barrier between Gaza and Israel and were fired on.
Hamas named those killed as Said Bassam Hellis, 21, Abdel Halim al-Fayyumi, 27 and Mohammed al-Fiqi, 24, AFP said.
The armed wing of Hamas, the al-Qassam Brigades, said on its website that the trio "were martyred during a jihad (holy war) operation".
The al-Qassam Brigades have said they will retaliate for today's killings, the BBC's Aleem Maqbool said from Ramallah.
The incident happened east of the Al-Bureij refugee camp.
Hamas's armed wing said on Tuesday that it was ending its five-month truce with Israel.
Earlier in the day the group launched a sustained barrage of rockets and mortars into Israel, the first attack of this magnitude since November.
The Israeli prime minister, Ehud Olmert, has ruled out a large-scale invasion of Gaza following the rocket attacks but he has promised some military response, our correspondent said.