GAZA CITY, Gaza Strip (AP) – Palestinian militants holding an Israeli soldier gave Israel less than 24 hours Monday to start releasing 1,500 Palestinian prisoners and implied he would die if it did not comply, but Israel said it would not negotiate.

Israeli Defense Minister Amir Peretz told Syria that he held it responsible for the fate of the captured soldier.

Cpl. Gilad Shalit’s captors are presumed to take orders from hard-line Hamas leaders based in the Syrian capital, Damascus.

“We will know how to strike those who are involved,” Peretz told a meeting of Israeli Labor Party officials.

The Israeli government said it would not cave in to extortion.

“There will be no negotiations to release prisoners,” the prime minister’s office said in a statement that held the Palestinians’ ruling Hamas party responsible for Shalit’s safe return.

However, Israel sent mixed messages, with its military chief, Lt. Gen. Dan Halutz, not ruling out talks for a prisoner swap.

The three militant groups that seized Shalit said that 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.”

“We give the Zionist enemy until 6:00 tomorrow morning, Tuesday, July 4, (11 p.m. EDT Monday)” the groups said in a statement posted on the Web site of the ruling Hamas party’s military wing and faxed to news agencies.

Israel fired artillery shells and missiles into the Gaza Strip and massed troops and tanks along the Gaza-Israel border.

But 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,” he said after leaving the Shalit family’s home in northern Israel.

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

Shalit, 19, was captured June 25 in a cross-border raid by Hamas’ military wing and two allied groups, the Popular Resistance Committees and the Army of Islam.

Israel has sent tanks, troops, gunboats and aircraft to attack Gaza over the past week to press militants to free Shalit. Intensive efforts to mediate his release, involving Egypt and other regional players, 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.

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 latest demand requires Israel only to “start” freeing the prisoners by Tuesday morning.

Hamas government spokesman Ghazi Hamad interpreted the ultimatum as “a message to Israel that all its military escalation will not get it anywhere.”

Hamas, which has killed hundreds of Israelis, 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.

Peretz nonetheless warned Damascus that he held it responsible for Shalit’s fate.

“I suggest that (Syrian President) Bashar Assad, who is trying to operate with his eyes shut tight, open his eyes, because he is responsible,” Peretz cautioned.

Last week, Israeli aircraft buzzed Assad’s summer residence to try to pressure him to lean on Mashaal to release Shalit. And Israeli Justice Minister Haim Ramon, a confidant of Prime Minister Ehud Olmert, said Mashaal, the target of a botched 1997 assassination attempt, remained in Israel’s sights.

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 hideout.

Wachsman’s mother, Esther, accused Israel’s leaders of a lack of candor in dealing with hostage 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.

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

Olmert told his Cabinet on Sunday that “I want nobody to sleep at night in Gaza” until Shalit is freed, according to an official present at the meeting.

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.

Israeli aircraft also struck Palestinian militants carrying anti-tank missiles near Israeli troops in northern Gaza, killing one, the army said.

Israeli troops killed one gunman after he and another militant approached soldiers in northern Gaza, the military said, without providing further details.

Israeli aircraft hit targets around Gaza including a building in Gaza City where the Al Aqsa Martyrs’ Brigades, a violent offshoot of Palestinian President Mahmoud Abbas’ Fatah movement, has an office, Palestinians and the military said.

No one was in the office and a family living on the first floor escaped harm.

The Palestinian parliament held its first session since Israel arrested dozens of top Hamas officials in the West Bank, including eight Cabinet ministers and 26 lawmakers, late last week. Lawmakers demanded that Israel release Palestinian prisoners as well as the arrested officials.

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