Israeli troops fired on a group of Palestinian youths in a Gaza refugee camp Saturday, killing three teenagers in the deadliest incident in the Gaza Strip (search) since Israel and the Palestinians declared a cease-fire two months ago.

The two sides offered dramatically different versions of the shooting, which shattered weeks of calm and added to tensions surrounding plans by Jewish extremists to march on a disputed holy site in Jerusalem on Sunday. The violence also came ahead of Prime Minister Ariel Sharon's departure Sunday for a meeting with President Bush (search).

Palestinian militants stopped short of calling off the cease-fire but said they reserved the right to respond to the shooting. In the hours after the incident, militants fired about 30 mortar shells at Jewish settlements in Gaza and fired at soldiers in southern Gaza, the army said. No one was injured in either case.

Palestinian leader Mahmoud Abbas said the Israeli shooting was a violation of the Feb. 8 truce. "We will not accept our children being killed in this way," said a statement issued by his office.

The shooting occurred in the Rafah (search) refugee camp, located in southern Gaza along the border with Egypt. Ali Abu Zeid, a 22-year-old Rafah resident, said the teens were playing soccer when the ball was kicked toward a border fence.

"The kids ran after it, and that's when we heard gunfire," he said. Palestinian hospital officials said two of the dead youths were 15, and the third was 14.

The Israeli army, however, said the youths were involved in smuggling across the border.

An Israeli commander at the scene said the youths were spotted crawling on their stomachs deep inside a closed military zone. He said three of the youths got to their feet and began running toward the border. The teenagers were about 250 yards into the closed area when they were shot on a military patrol road, he said.

"These kids were not playing soccer," said the commander, whose name was withheld under military regulations. "You don't get there by accident."

Tawfiq Abu Khoussa, a spokesman for the Palestinian Interior Ministry, called the Israeli account "completely false."

Rafah has been a frequent flashpoint of fighting, although violence has dropped sharply since Abbas and Sharon declared the cease-fire. Last month, Palestinian militant groups joined the truce.

A total of 13 Palestinians have been killed since the truce went into effect. Five Israelis have also died during the period, killed in a Feb. 25 suicide bombing in Tel Aviv.

Hamas, the largest Palestinian militant group, pledged to avenge the deaths of the three teens.

"The Palestinian people cannot stay silent in the face of this crime and it cannot pass without punishment," said Saeed Siyam, a Hamas leader in Gaza. He would not comment on whether Hamas remained committed to the truce.

Mohammed al-Hindi, leader of Islamic Jihad, said his group would continue to honor the truce, but reserved the right to retaliate. Later, Islamic Jihad militants claimed responsibility for firing the mortar shells at Jewish settlements in Gaza.

Tensions were already high amid plans by Jewish extremists to rally Sunday at a disputed holy site in Jerusalem.

Palestinian militants have threatened to end the cease-fire if the rally proceeds, and Israel's public security minister, Gideon Ezra, said Saturday that police would prevent the Jewish protesters from reaching the site. Police said they would restrict access to Muslims over the age of 40.

The hilltop area, known to Muslims as the Noble Sanctuary and to Jews as the Temple Mount, is currently home to the Al Aqsa Mosque compound.

Despite the warnings, Revava, the group sponsoring the march, said it planned to proceed with the event.

Extremist Jewish groups opposed to Sharon's planned withdrawal from the Gaza Strip this summer have threatened to storm the site in hopes of sparking Muslim riots and disabling the pullout plan.

David Haivri, a Revava leader, said his group was only interested in asserting its right to pray on the Temple Mount. Access has been restricted since Israel captured east Jerusalem in the 1967 Mideast war.

The latest tensions threatened to cloud Sharon's meeting with Bush at the president's Texas ranch Monday. The two men are expected to discuss the Gaza withdrawal plan.