Three Palestinian militant groups that captured an Israeli soldier demanded Monday that 1,500 Palestinian prisoners start being released in less than 24 hours, implying that he would be killed, but Israel said it would not negotiate.

"There will be no negotiations to release prisoners," the office of Prime Minister Ehud Olmert said in a statement that held the Palestinians' ruling Hamas party responsible for the safe return of Cpl. Gilad Shalit.

Tensions over the kidnapping of an Israeli soldier continued to boil Monday. Three Palestinian militant groups gave Israel less than 24 hours to release 1,500 prisoners, implying that Cpl. Gilad Shalit would be killed. But Israel said it would not negotiate over Shalit, shown here (foreground) in this photo made available by his family last week. [AP]

Also, Israeli Defense Minister Amir Peretz told Syria on Monday that he held it responsible for the fate of a captured Israeli soldier.

"We will know how to strike those who are involved," Peretz said.

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.

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.

"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.

"If the enemy does not respond to our humanitarian demands mentioned in previous leaflets on the conditions for dealing with the case of the missing soldier ... we will consider the soldier's case to be closed," it said. "And then the enemy must bear all the consequences of the future results."

Shalit, 19, 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.