Hamas celebrated its 21st anniversary with a mass rally last weekend
The Islamist militant group Hamas says it has ended its six-month ceasefire with Israel in the Gaza Strip.
As the ceasefire expired at 0400 GMT, Hamas issued a statement blaming Israel which had not "respected" the truce.
Israel's foreign ministry spokesman said the militants, who control Gaza, "had chosen violence over truth".
The Egyptian-brokered deal began on 19 June but has been tested regularly by Palestinian rocket attacks on Israel and Israeli operations in Gaza.
Hours after the announcement, Palestinian television carried pictures of militants conducting exercises.
The Israeli military reported a minor shooting incident in the fields of a kibbutz farm near the Gaza border and two rockets exploding in southern Israel, but there were no reports of casualties.
Israel's Haaretz newspaper reported that the Israeli military had cancelled weekend leave for all troops stationed near the Gaza Strip and instructed units to prepare for mobilisation.
Hamas blamed Israel for the end of the ceasefire on Friday, saying it had not respected its terms, including the lifting of the blockade under which little more than humanitarian aid has been allowed into Gaza.
Israeli foreign ministry spokesman Yigal Palmor on the end of the truce
Israel said it initially began a staged easing of the blockade, but this was halted when Hamas failed to fulfil what Israel says were agreed conditions, including ending all rocket fire and halting weapons smuggling.
Israel says the blockade - in place since Hamas took control of Gaza in June 2007 - is needed to isolate Hamas and stop it and other militants from firing rockets across the border at Israeli towns.
The UN's relief agency says the situation has created a "profound human dignity crisis".
Announcing the end of the truce on its website, Hamas said: "The ceasefire is over and there won't be a renewal because the Zionist enemy has not respected its conditions."
Israeli foreign ministry spokesman Yigal Palmor said Hamas had chosen "violence over truth and rocket-shooting over ceasefire".
He said it showed that Hamas "does not have the best interest of Palestinians in mind".
"We have said publicly on many occasions that we think the continuation of the ceasefire is in the best interests both of Israelis and of Palestinians," he said.
Palestinian Authority President Mahmoud Abbas, the head of Fatah which controls parts of the West Bank, is in Washington to discuss the situation with US President George W Bush.
The Palestinian leader will go to Moscow on Saturday, where he will meet President Dmitry Medvedev for the first time.
Russia is a member of the Middle East diplomatic "Quartet", which also includes the European Union, the UN and the US.
The US state department issued a statement following the Hamas announcement, saying that the ceasefire should be "respected and extended".
"Violence will not advance, but retard, the hope for peace between Israelis and Palestinians, which currently is being pursued by the legitimate Palestinian government of President Abbas," the statement said.
The BBC's Arab affairs analyst Magdi Abdelhadi says the end of the ceasefire comes at a critical moment for the Israelis and the Palestinians.
The Israelis are in the middle of an election campaign, and political turmoil on the Palestinian side is set to worsen amid intensified rivalry between Hamas and Fatah.
Hamas, which controls Gaza, is religiously conservative and more hardline in its view of the conflict with Israel, feeling peace talks have achieved little.
Fatah is a secular party which favours dialogue with Israel, but was voted out of government in 2006 in favour of Hamas.
Our correspondent says the failure to extend the truce is hardly surprising, given the fact that the deal has largely failed to achieve what each side originally wanted from it.
Israel thought that it could lead to the release of Cpl Gilad Shalit, captured by militant groups over two years ago.
Hamas hoped it would give it breathing space to consolidate its grip on the Gaza Strip and end the joint Israeli-Egyptian blockade.
The Egyptians hoped it could help to end the deepening rift between Hamas and Fatah.
However, analysts say that many Palestinians feel the fight between the two factions has become less about ideology, but more about power, control and, ultimately, revenge.
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