The Israeli prime minister has rejected an ultimatum from militant groups calling for the release of 1,500 Palestinian prisoners in an implied exchange for the life of an Israeli soldier, FOX News has learned.

Prime Minister Ehud Olmert's office released a statement Monday that reiterated the country's non-negotiation stance: "We will not conduct any negotiations on the release of prisoners. The Palestinian Authority bears full responsibility for the welfare of Gilad Shalit and for returning him safe and sound to Israel."

Three Palestinian militant groups that captured the 19-year-old Shalit issued a statement Monday giving Israel less than 24 hours to start releasing 1,500 Palestinian prisoners.

If Israel doesn't comply with the militants' demands, "we will consider the soldier's case to be closed," it said, an apparent reference to killing him. "And then the enemy must bear all the consequences of the future results."

The ultimatum came as Israel made good on its promise to continue its military offensive until the soldier was freed, firing artillery shells and missiles into the coastal strip and massing troops and tanks along the Gaza-Israel border.

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The militant groups, in a statement posted on the Web site of the ruling Hamas party's military wing, did not expressly say what the consequences would be, but implied the soldier could be killed. Israeli government spokesman Asaf Shariv said officials were studying the statement.

"We give the Zionist enemy until 6:00 tomorrow morning, Tuesday, July 4," the groups said in their statement, which was also faxed to news agencies.

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Cpl. Shalit was captured June 25 in a cross-border raid by the military wing of the Palestinians' ruling Hamas party, and two allied groups, the Popular Resistance Committees and the Army of Islam.

Olmert has said repeatedly that Israel would not negotiate Shalit's release. But military chief Lt. Gen. Dan Halutz did not say no when asked by reporters if Israel should negotiate.

"We, and by that I mean the political and military echelons, will consider all that there is to be considered, then reach conclusions and act on them," Halutz said after leaving the Shalit family's home in northern Israel.

Israel has swapped prisoners before to win the release of captured citizens, alive and dead, in usually lopsided deals that hand far more prisoners to the Palestinians.

In the meantime, Israel has sent tanks, troops and warplanes to attack Gaza over the past week in an effort to press militants to free Shalit. Intensive efforts to mediate his release, involving Egypt and other countries, so far have not been successful.

There has been no sign of life from the soldier since his seizure, and no concrete evidence of his condition, though Israeli officials have said they think he is alive. The Shalit family had no immediate comment on the ultimatum.

The pan-Arab newspaper Al Hayat reported Monday that an Egyptian security team had visited the kidnapped Israeli soldier in Gaza and said he was being treated by a Palestinian doctor for "three bullet wounds." The paper did not say when the visit took place.

Shalit's captors initially demanded the release of about 500 women and children prisoners held in Israeli jails. They later raised their demands to include an additional 1,000 prisoners. Israel is currently holding about 9,000 Palestinians.

The ultimatum requires Israel only to "start" freeing the prisoners by Tuesday morning.

Hamas, which has killed hundreds of Israelis in suicide bombings, has refused to renounce violence or recognize Israel since taking power in March. But the Hamas government and Hamas leaders based in Syria have denied responsibility for the soldier's capture.

A spokesman for the Hamas government said the ultimatum was "a message to Israel that all its military escalation will not get it anywhere."

"If it continues every day to kill and target and attack, it won't get the soldier, alive or dead," spokesman Ghazi Hamad said. "That is the meaning of the message."

In their statement, Shalit's captors accused Israel of not "learning lessons" from the cases of other kidnapped soldiers. The last Israeli soldier kidnapped by Hamas, Nachshon Wachsman, died in 1994 in an Israeli commando raid on his captors' Jerusalem hide-out.

In an op-ed piece in the Haaretz newspaper, Wachsman's mother, Esther, wrote how the family, through Shalit's capture, was "once again going back" to those dark days when her son was being held. She criticized Israel's leaders for a lack of candor in dealing with abduction cases.

"I am not calling for the release of murderers, but they [Israel's leaders] should not insult our intelligence because they have negotiated and they have given in to terror," Esther Wachsman wrote, referring to cases in which Israel has swapped prisoners in the past.

When it launched its first large-scale military action in Gaza since withdrawing from the strip last summer, Israel said it declared purpose was to lean on militants to release Shalit. In statements since, government officials have said the campaign is also meant to topple the Hamas government and stop gunmen from launching rockets at southern Israel.

Early Monday, Israel massed tanks and troops across from northern Gaza, and pounded the area with artillery. At daybreak, a small force of Israeli tanks entered northern Gaza, but the military said it was a "limited" mission to find explosives and tunnels near the border fence.

Additional Israeli troops moved into place across from northern Gaza on Sunday, showing clear preparations for an invasion that was put off last week to give diplomacy more time. For months, Palestinian militants have launched homemade rockets at Israeli villages near the border fence, and Israel has been unable to stop the barrages with repeated airstrikes and artillery attacks.

Also Monday, Israeli aircraft struck Palestinian militants carrying anti-tank missiles near Israeli troops in northern Gaza, killing one, the army said. Earlier, Israeli troops killed one gunman after he and another militant approached soldiers in northern Gaza, the military said, without providing further details.

Palestinian medical officials confirmed that an armed Palestinian man had been killed in Beit Hanoun.

Despite the military operation, Israel reopened the main cargo crossing with Gaza Sunday to allow 50 trucks of food, medical supplies and fuel into Gaza from Israel. Trucks carrying diesel fuel, gasoline and natural gas also began entering northeastern Gaza through the Nahal Oz border crossing.

The Palestinian parliament, meanwhile, held its first session Monday since Israel arrested 64 top Hamas officials, including eight Cabinet ministers and 20 lawmakers, last week in the West Bank.

"By arresting those lawmakers and ministers, Israel is trying to hijack the Palestinian ... political regime, but our people will protect our political regime," said parliament speaker Abdel Aziz Duaik.

Prime Minister Ismail Haniyeh of Hamas, whose empty office was hit by Israeli missiles on Sunday, did not attend the session, which was convened to discuss the arrests.

The Associated Press contributed to this report.