The Palestinian Islamist movement Hamas which controls the Gaza Strip said on Sunday that a troubled Cairo-brokered truce with Israel will not be renewed when it runs out later this week. But a spokesman for outgoing Israeli Prime Minister Ehud Olmert insisted his government remained keen to see the six-month-old truce extended beyond Thursday provided Hamas halted rocket and mortar fire against southern Israel.

"The truce was limited to six months and ends on December 19," Hamas political leader Khaled Meshaal said in a television interview from Damascus with Hamas' Al-Quds satellite television.

"Given that the enemy is not respecting its commitments and the blockade is still in place against our people, for Hamas, and I think for the majority of forces, the truce ends after December 19 and will not be renewed," he said.

Meshaal's statement came on the day marking the 21st anniversary of the Islamist group's formation at the start of the first Palestinian uprising.

Israel began its crippling siege of the impoverished territory after Hamas routed the rival Fatah movement in parliamentary polls deemed fair and democratic by international observers in 2006. The Jewish state further tightened its chokehold on Gaza after Hamas pre-empted what many have described as a US- and Israeli-backed offensive by Fatah aimed at ousting the Islamists from the enclave in 2007.

Various UN and EU officials have slammed the blockade, saying it only strengthens Hamas and amounts to "collective punishment of a civilian population," an illegal act that the Fourth Geneva Convention defines as a war crime.

On Wednesday, the top UN human rights envoy to Palestine said Israeli policies amounted to a "crime against humanity."

Under the terms of the Egyptian-mediated truce that went into effect in and around the Gaza Strip in June, Israel was to significantly ease its siege if Hamas reigned in militants launching attacks on the Jewish state. But while the Islamists virtually halted all rocket attacks, Israel never honored its commitment.

However, the cease-fire largely held until November 4 when Israel invaded the Gaza Strip with tanks and soldiers in an offensive that killed seven Hamas members. Following Israel's breaking of the truce, Gaza-based fighters resumed rocket fire into the Jewish state.

UN officials have dismissed Israel's official line that rocket fire is forcing them to completely seal off the strip, pointing out that in past times of far greater violence, international humanitarian aid was always let in.

Senior Israeli Defense Ministry official Amos Gilad, who conducted the negotiations for the original truce, returned to Cairo on Sunday for talks with Egyptian mediators on an extension. Neither he nor Egypt's pointman for the negotiations - intelligence chief Omar Suleiman - made any comment after their talks.

But the Israeli premier's spokesman, Mark Regev, later said in Occupied Jerusalem: "Israel is interested in calm reigning in the south. It was and is still ready to respect the commitments obtained through the mediation of Egypt."

Gilad - a reserve major general and key aide of Defense Minister Ehud Barak - has been an outspoken defender of the Gaza truce despite the shattering of the agreement since Israel's November 4 raid.

"Experience shows that military operations don't always solve problems in the Middle East," he said last month. "You have to find the optimal solution. To date no appropriate military solution has been found for the Strip."

But Foreign Minister Tzipi Livni said the Jewish state was determined to end the rule of Hamas rule in Gaza.

"The state can and should provide an answer to the terror with its available military means. We can not allow Gaza to remain under the control of Hamas," Livni's office quoted her as saying in a statement.

Hamas had shown mounting frustration with the truce agreement since Israel sharply tightened its blockade of Gaza after breaking the deal last month.

For Sunday's anniversary, Hamas staged a show of strength, drawing huge crowds on to the streets of Gaza City that Hamas television said ran into the hundreds of thousands.

In an address to the crowds, the head of the Hamas government in Gaza, Ismail Haniyya, boasted that US President George W. Bush's administration had failed to defeat his movement, which had only grown stronger.

"Bush declared war on the Palestinian people ... He provided money and arms to the seditionists to wage a war against legality," Haniyya said, referring to the deadly street fighting with loyalists of Western-backed President Mahmoud Abbas that preceded Hamas' takeover of Gaza in June 2007.

"Bush failed, we have not been overthrown," he said. And despite Israel's blockade, "Hamas is stronger and will remain stronger because it draws its strength from God."

Meanwhile, Israel said on Sunday it was keeping border crossings with Gaza sealed off after rocket fire from the impoverished territory.

"This decision was taken after rocket and mortar fire on Saturday toward southern Israel," said Peter Lerner, spokesman for the coordinator of Israeli activities in the Palestinian territories.

The rockets fired from Gaza caused no victims or damage, Israeli police said