A pair of 17-year-old Palestinians killed two Israelis in homicide bombings Tuesday, rattling a shaky Mideast truce and prompting warnings by Israel that it will freeze a U.S.-backed peace plan unless all violence stops.

The Islamic militant group Hamas and renegades from Yasser Arafat's Fatah movement (search) dispatched the teens, one to a grocery store in the Tel Aviv suburb of Rosh Haayin and the other to a bus stop outside the Jewish settlement of Ariel, about 10 miles to the east, in the West Bank (search).

"We heard the blast and things started to fly, electrical cords, glass, [there were] horrid screams," said shopper Juliet Tiav, who was injured.

The explosions killed a 42-year-old man from Rosh Haayin, who had been buying breakfast groceries for his two children, and an 18-year-old soldier standing at the Ariel bus stop. Eleven bystanders were wounded.

The young assailants, street vendors who grew up a few blocks from one another in the West Bank city of Nablus (search), blew themselves up less than an hour apart, but an Israeli army commander said there apparently was no coordination. Relatives said the two didn't know each other.

Hamas said the Ariel bombing came as revenge for the killing of two of its bomb makers in an Israeli army raid on Friday, and that the group would now return to observing a three-month truce it declared June 29. The Fatah-linked Al Aqsa Martyrs' Brigades (search), who claimed responsibility for the attack near Tel Aviv and opposes the cease-fire, threatened more attacks.

Israel said the bombings were proof Palestinian Prime Minister Mahmoud Abbas and his security chief, Mohammed Dahlan, have been shirking their obligation under the U.S.-backed "road map" peace plan to dismantle militant groups.

"Israel won't be able to continue the [peace] process, despite its strong desire, if terrorism does not cease completely and the Palestinian Authority does not fulfill all its commitments," said Israeli Prime Minister Ariel Sharon.

However, an Israeli military source said Israel would not launch a large-scale strike in retaliation for the bombings, apparently to avoid being blamed for the possible collapse of U.S. peace efforts. Sharon was meeting later Tuesday with a senior U.S. envoy, State Department official William Burns.

In Washington, Secretary of State Colin Powell said peace efforts would continue, but "we will never get there so long as people continue to participate in terrorist activities, and we see the response to terrorist activities, which are necessary for self-defense."

Troops sealed off Nablus and imposed a curfew on surrounding West Bank villages. Sharon also delayed the release of 76 Palestinian prisoners, none involved in anti-Israeli violence, who were to have been freed Tuesday. They had already boarded buses when they were ordered back to jail.

Abbas, who says he will not confront the militant groups for fear of setting off a civil war, insisted the Palestinian factions remain committed to the truce. "The escalation of the situation came from the Israeli side," Abbas said in Qatar, before heading to Amman, where he is to meet with Burns on Wednesday.

In a statement, the Palestinian leadership said it had "always condemned targeting civilians," but held Israel responsible for the rise in tensions.

Interviewed on Israel TV, Elias Zananiri, Dahlan's spokesman, denounced the attacks. "All the excuses [by the militants], like the Israeli army raid in Nablus a few days ago, don't matter. This is unacceptable," he said.

Violence has continued even after the declaration of the cease-fire by Hamas, Islamic Jihad and Fatah. Seven people have been killed on the Israeli side and 15 people -- including three homicide bombers -- on the Palestinian side. Israel's military has counted 120 shooting incidents in the West Bank and 10 mortar attacks on Israeli targets in the Gaza Strip. Troops, in turn, keep carrying out arrest raids.

Tuesday's were the first homicide bombings since a Palestinian from a splinter cell within the Islamic Jihad group blew himself up in a house on July 7, killing a 65-year-old woman. Still, the past six weeks had been relatively calm, compared to the intense fighting that broke out in September 2000.

The first homicide bomber struck Tuesday in a grocery store in Rosh Haayin shortly after 9 a.m. The blast set a fire and left a mass of twisted blinds and shattered glass.

The bomber, 17-year-old Khamas Gerwan, and an Israeli shopper, 42-year-old Yehezkel Yekutieli, were killed and nine bystanders were wounded.

Gerwan was a Fatah follower who had peddled shoes and left his home in the Askar refugee camp on the edge of Nablus on Monday.

The second assailant, Islam Qteishat, a young Hamas recruit, was also a street vendor, selling books, pens and notepads. He detonated 22-33 pounds of explosives at the entrance to Ariel, killing soldier Erez Hershkovitz, 18.

The bomber's mother, Yusra, 40, demanded retribution against the militants who sent her son to his death -- a rare attitude in a society where homicide bombers are usually revered as martyrs. "I'll kill whoever dispatched my son," she screamed, beating her fists against a wall.

The parents of both bombers said they didn't know each other, though the families live on the same street.

A political leader of Hamas in Gaza, Ismail Hanieh, suggested that now that the score had been evened in Hamas' view, the group would return to observing the cease-fire. Asked whether Hamas was calling off the truce, he said: "No, this is a reaction to the continuous violation [of the truce] by Israel."