Jerusalem ? Three Palestinian teenagers were shot and killed by Israeli soldiers Saturday in the southern Gaza Strip, and Palestinian guerrillas responded by firing 25 mortar shells at Jewish settlements in Gaza, threatening to unravel a tenuous, de facto truce in place for more than two months.

A senior leader of the radical group Islamic Jihad told reporters in Gaza, however, that the undeclared cease-fire was still in effect.

“So far, the Palestinian factions have not declared an end to the calm … but they are studying the issue anew because of the continuing Zionist aggression,” Islamic Jihad leader Mohammed Hindi said at a news conference in Gaza.

Palestinian witnesses said Saturday’s incident occurred at about 5 p.m. when a group of youths who were playing soccer in the Rafah refugee camp, near the Egyptian border area in southern Gaza, were fired upon by Israeli troops. The witnesses said the troops fired for no reason, killing three youths: Ahmed Jazzar, 14, Khalid Ghannam, 15, and Ashraf Mousa, 16. An Israeli military spokeswoman said that the shooting occurred in a border area where Palestinians are not allowed to be, in a spot that is an established route for smuggling weapons from Egypt into Gaza.

She said that Israeli soldiers patrolling the border in a jeep spotted a group of five Palestinian men crawling on the ground and that three suddenly broke into a run toward the border. She said the soldiers fired warning shots, and when the three men did not respond, the soldiers shot them.

“This area is completely prohibited” to civilians and far away from any homes, she said. “No one enters there unless they want to carry out some kind of attack.”

The president of the Palestinian Authority, Mahmoud Abbas, announced an end to hostilities against Israel at a Feb. 8 summit with Israeli Prime Minister Ariel Sharon in the Egyptian resort of Sharm el-Sheik. Israel did not agree to a truce, but said it would cease military activity. The mutual declarations have led to a tranquil period after 4 1/2 years of conflict.

March was the first month without any Israeli deaths from Palestinian attacks since the start of the Palestinian uprising in September 2000. On Feb. 25, five Israelis were killed and about 50 were wounded when a Palestinian suicide bomber blew himself up outside of a crowded nightclub in Tel Aviv.

For Palestinians, Saturday’s incident was the deadliest since Jan. 15, when eight Palestinians were killed in three separate Israeli attacks in Gaza.