UPDATE 12:40 p.m. A cease-fire is set to begin at 2 p.m. E.T ( 9 p.m. local time) between Israel and Hamas, according to US Secretary of State Hillary Clinton and Egypt's Foreign Minister Mohammed Kamel Amr.

A bomb explosion rocked a commuter bus in downtown Tel Aviv this morning, injuring about two dozen people as militants tried to widen attacks on Israel. The bombing is likely to dim international mediators' efforts to finalize a cease-fire between Israel and Hamas after a week of fighting.

The explosion was the first bombing in Israel’s economic and cultural capital in six years. It comes after a week that saw rockets fired from the Gaza Strip reach the area around Tel Aviv for the first time. Those caused no casualties in the city because they either were intercepted by Israel’s Iron Dome Rocket system, or fell in open areas.

Hamas had vowed to open a bombing campaign against Israel in retaliation for the offensive that started Nov. 14 with the killing of its top military chief. But it was initially unclear exactly who is responsible for the bombing. Preliminary reports indicated that one or several bombers threw an explosive on the bus and fled the scene, prompting Israeli police to begin a manhunt in response.

The bombing blew out the windows of the public bus just outside the walls of the office complex housing Israel's military main command, tipping the national mood away from an anticipated cease-fire deal and in favor of continuing an offensive against Hamas and other militant groups in Gaza.

"I was passing by the bus station when, all of a sudden, I heard a boom. Then I saw smoke and thought, 'It’s definitely an attack'," says Golan Tuviya, a passerby who was just a few feet away from the explosion. "Then I just fled."

Politicians from the coalition of Prime Minister Benjamin Netanyahu called on the Israeli leader to abandon truce efforts and redouble efforts to hit Palestinian targets to stop a rain of hundreds of rockets into Israeli cities.

"There won’t be a cease-fire. You don’t understand," says Riva Rothberg, an onlooker who came to the scene of the bombing. "It’s an issue of blood vengeance. They hit us and we hit them."

Despite that sentiment, Israel Radio reported that cease-fire talks were continuing. The explosion came as US Secretary of State Hillary Clinton held meetings in Cairo to bolster the cease-fire efforts. Last night, amid elevated hopes for an announcement, she met with Israeli Prime Minister Benjamin Netanyahu. She also met with Palestinian Authority President Mahmoud Abbas this morning.

Though it is extremely unlikely that the perpetrators came from Gaza, which is walled off by a military blockade, analysts said it was possible that the order was issued by militants in the coastal strip and carried out by colleagues in the West Bank.

The attack would ostensibly boost claims by Palestinian militants that they ended the week of violence with the upper hand. Although Israeli and Palestinian security forces have nearly eliminated the capabilities of militants in the West Bank in recent years, "residual capability" still exists, says Gerald Steinberg, a political science professor at Bar Ilan University.

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"The working assumption has to be this is from Hamas" or other allied militant groups, he says.

"This was clearly ad hoc – someone throwing a bomb into largely empty bus. It has none of the hallmarks of the bombing campaign of 2002 and 2003, but the timing is most likely part of the Hamas campaign."