A six-month Egyptian-brokered truce between Israel and Palestinian factions, led by Hamas Islamists, in the Gaza Strip ended on Friday, raising the prospect of fresh cross-border fighting.
"We announce that the calm between us and the Zionist enemy has finished entirely and it will not be renewed as a result of the occupation's denial of its fundamental conditions and obligations," said Hamas's armed wing, Izz el-Deen al-Qassam, in a statement issued at 0400 GMT, which Hamas said was the official deadline for the truce to expire.
Hamas and other militant groups in the Gaza Strip said they were prepared for any military escalation with Israeli forces and possible Israeli army raids into the territory. They said their gunmen had undergone training during the six-month truce.
Qassam Brigades said any Israeli incursion would "open the doors of the battle wide and would be met by a painful response from our side."
"The calm is over," Hamas official Ayman Taha said in an announcement after concluding talks with Palestinian factions on Thursday in the coastal enclave controlled by the Islamist group.
He said the ceasefire would not be renewed "because the enemy did not abide by its obligations" to ease a crippling blockade of the Gaza Strip and halt all attacks.
Hamas stopped short of threatening an immediate escalation of violence against Israel, which had hoped to extend the truce and appears wary of a confrontation that could cause heavy casualties on both sides.
"We think the lull is in the best interest of both sides," said Israeli Foreign Ministry spokesman Yigal Palmor.
"We would like it to continue. If Hamas chooses violence over the ceasefire, rocket shooting over the possibility of improving the situation in Gaza, then one must ask oneself whether Hamas has the best interests of its people in mind or whether there are foreign interests that are involved."
Palmor did not name foreign influences, but Hamas gets support from Iran and Hamas leaders live in exile in Syria.
Mark Regev, spokesman for Israeli Prime Minister Ehud Olmert, said more than 30 rockets launched from Gaza into Israel over the weekend were "the opposite of calm".
Israel is willing to abide by the truce terms but "every rocket launched is a direct and flagrant violation of the understandings reached and cannot be tolerated," he said.
The European Union, in a statement, called for an immediate cessation of both rocket fire and Israeli incursions.
Tensions along the Israeli-Gaza border have been escalating since early last month when a deadly Israeli army raid triggered a wave of rocket attacks by Palestinian militants.