Militants in the Hamas-controlled Gaza Strip fired at least two rockets into southern Israel, breaching a five-day-old ceasefire after Israeli troops killed two Palestinians in the occupied West Bank.
The Egyptian-brokered truce, which took effect last Thursday, calls on Hamas to prevent cross-border rocket fire from the Gaza Strip, which it seized by force a year ago.
The deal, under which Israel agreed to halt its own attacks in the Gaza Strip and to ease its economic blockade of the impoverished coastal enclave, does not apply to the West Bank.
An Israeli police spokesman said one of the makeshift rockets fired from the Gaza Strip hit a house in the border town of Sderot, causing damage but no casualties. The second rocket fell in an open area.
There were no immediate claims of responsibility for the attack, just hours after Israeli Prime Minister Ehud Olmert thanked Egyptian President Hosni Mubarak in talks in Egypt for brokering the ceasefire.
The salvoes followed Israel's killing overnight of two Palestinians, including an Islamic Jihad commander, in the West Bank city of Nablus.
It was the first fatal raid since a ceasefire took hold in the Gaza Strip last Thursday. Similar West Bank operations and Palestinian rocket attacks from the Gaza Strip led to the break down of previous truce deals.
Islamic Jihad had threatened to launch attacks inside Israel to avenge the death in Nablus of Tarek Juma Abu Ghali, whom the militant group described as one of its most senior commanders in the northern West Bank.
A second Palestinian, affiliated with the Islamist militant group Hamas, was also killed in the raid in Nablus.
"Calm in Gaza does not mean that we will sit in our seats waiting to be slaughtered one by one," Islamic Jihad said in a statement. "This crime will not pass without punishment and the coming days will be a witness to that."
Hamas, which claimed responsibility for a shooting attack that wounded three Israeli hikers near a West Bank settlement on Friday, also called on Palestinian groups in the West Bank to retaliate for the killings.
Nablus Governor Jamal Muheisen called the Israeli raid in the city an "unjustified crime" but said he did not believe it would threaten the Gaza truce.
Officials on both sides said from the start that they doubted the truce in the Gaza Strip would last.
Earlier on Tuesday, the Israeli army confirmed that Palestinians fired a mortar shell into Israel from Gaza overnight in the first reported violation by militants of the ceasefire.
No one was hurt in that incident and there was no immediate claim of responsibility.