Mark Regev, an Israeli spokesman, said: "Hamas is directly responsible for the escalation in the violence.
"Israel had wanted to see the calm that prevailed until recently prevail once again. But Hamas ... are seeking a dangerous escalation."
But Mahmoud al-Zahar, a senior Hamas leader, said "self-defence and resistance" would continue and told Israel that if they wanted to continue the truce, "then abide by it".
"Up to this moment we are committed to the ceasefire," he said.
"But if the Israelis decide to go away from the ceasefire we are ready ... We are waiting for the Israelis. If they are committed really we have to address that frankly."
One Israeli in Sderot was treated for shrapnel wounds, and a number of people were treated for shock.
Micha Ben Alain, whose kibbutz (collective farm) near Sderot was hit, said: "Fortunately there were no casualties but there was a lot of damage.
"Windows were smashed and doors were torn apart".
Israel's closure of Gaza's border crossings have halted emergency UN food and fuel supplies to the territory.
A senior Israeli defence official on Friday said that "due to the continued rocket fire the crossings are shut today ... there is no intent to open them today".
Christopher Gunness, a spokesman for the United Nations Relief and Works Agency for Palestine Refugees (UNRWA), said: "People are going to start getting hungry."
The European Union urged Israel on Friday to respect international law by allowing essential services into the Gaza Strip as its blockade takes a heavy toll on the impoverished Palestinian territory.
|Israel is refusing to allow emergency food and fuel supplies to Palestinians in Gaza [AFP]
Benita Ferrero-Waldner, the EU External Relations Commissioner, said: "International law requires the provision of access to essential services such as electricity and clean water to the civilian population.
"I am profoundly concerned about the consequences for the Gazan population of the complete closure of all Gaza crossings for deliveries of fuel and basic humanitarian assistance," she said.
"I call on Israel to re-open the crossings for humanitarian and commercial flows, in particular food and medicines. Facilitation of fuel deliveries for the Gaza power plant should be resumed immediately."
Israel also cut off European Union-funded fuel supplies to Gaza's sole power plant on Thursday, prompting it to shut down for want of diesel.
Humanitarian agencies warned that the continued closure would lead to a further deterioration of the already precarious situation in Gaza.
The Israeli government generally responds to Gaza attacks by sealing off the territory, cutting off the already limited basic supplies it allows in under the blockade it imposed after Hamas seized control in June 2007.
Clashes began on November 4 when Israeli forces killed six Hamas fighters in a raid to destroy a secret supply tunnel.
Israeli troops shot dead four Hamas gunmen in a further incursion on Wednesday, and Hamas responded with more rocket and mortar attacks, ratcheting up tensions in what Ayman Taha, a Hamas spokesman, called a "dangerous escalation".
Short of fuel, Palestinian officials shut down Gaza's sole power plant on Thursday, causing partial blackouts in Gaza City.
European Union-funded fuel generates about a third of the electricity consumed by Gazans. The rest comes from Israel.
Israel had initially said it would allow 30 trucks of food and other humanitarian goods into the enclave, but they were turned back on Thursday.