Jerusalem ? A suicide bomber blew himself up Tuesday on a bus packed with Jewish worshippers returning from the Western Wall, killing at least 18 people.

Hamas and Islamic Jihad both claimed responsibility for the blast, one of the deadliest Palestinian attacks in the past three years.

More than 100 people were also injured, 40 of them children, hospital officials said.

The attack marked perhaps the most serious blow yet to the U.S.-backed “road map” peace plan, which was unveiled three months ago. It shattered a truce called by militants on June 29 that had been fraying in recent weeks with less deadly attacks.

In an immediate response, Israel froze all contacts with the Palestinian Authority, as well as the handover of two West Bank towns, Jericho and Qalqiliya, to Palestinian control. The handover had been expected later this week.

Israel also decided to seal off the West Bank and Gaza Strip, a security source said.

The powerful explosion ripped through the tandem bus — which has two passenger sections — shortly after 9 p.m. (1 p.m. CDT). At the time, the bus was heading from the Western Wall, Judaism’s holiest shrine, to an ultra-Orthodox Jewish neighborhood on the outskirts of Jerusalem. Many families with children were on the bus, witnesses said. Five children were among the dead.

“I had just come home from praying at the Western Wall and was heading home,” said Zvi Weiss, an 18-year-old Jewish seminary student from New York City who sat in the front of the bus and escaped unharmed.

“The bomb went off at the back of the bus. Everything went black. I climbed out of the broken window and started running. All around me there were people covered in blood, screaming, some with limbs missing.”

The Palestinian Authority decided to cut all dialogue with Islamic Jihad and Hamas and instead use security forces to take action against the groups in the coming days, a Palestinian official said on condition of anonymity. It was unclear what sort of action the Palestinians planned.

Palestinian Prime Minister Mahmoud Abbas had been meeting with Islamic Jihad leaders in the Gaza Strip at the time of the explosion, in a renewed attempt to persuade them to halt attacks.

Abbas condemned the bombing as a “terrible act” and said he ordered Palestinian security forces to investigate. Abbas has been trying to use persuasion, rather than force in handling the militants. It might cost him his job if his approach fails and violence continues.

In Tuesday’s bombing, at least 18 bystanders were killed, including five children, the Zaka rescue service said. Police initially had put the death toll at 20.

Police said the powerful bomb had been packed with bits of metal, for greater deadliness.

The attack followed a deadly explosion set off by a suicide bomber outside the hotel housing the U.N. headquarters in Baghdad.

An Israeli government spokesman said there was no known direct link between the two bombings, although he said the motivations were similar.

“It is motivated by extremist Islamist militants who don’t accept the legitimacy of the West or of Israel,” said the spokesman, Dore Gold.

Last week, Islamic Jihad threatened attacks on Israelis to avenge the killing of a senior operative, Mohammed Sidr, in an Israeli arrest raid in the West Bank city of Hebron.

In a phone call to The Associated Press, Islamic Jihad claimed responsibility for Tuesday’s bombing, saying it was in revenge for the killing of Sidr, whom Israel had accused of plotting a series of attacks.

However, later Tuesday, Hamas distributed fliers in Hebron, saying the Jerusalem bombing was carried out by one of its supporters, identified as Raed Abdel-Hamed Mesk, 29, a mosque preacher from Hebron.