At least 21 people have been injured in an explosion on a bus in Israel's commercial capital, Tel Aviv, in what police described as a "terrorist attack".
The blast shattered windows on the bus as it drove along a tree-lined street next to Israel's huge defence ministry complex on Wednesday. Emergency services said five of the wounded were in a serious or moderate condition.
Police said it was not a suicide attack and suggested that someone might have left the device on the bus. "We do believe it was a terrorist attack," Israeli police spokesman Mickey Rosenfeld told Al Jazeera.
The bombing happened on the eighth day of an Israeli offensive against the Palestinian Gaza Strip and coincided with intensive diplomatic efforts to secure a lasting truce.
Fawzi Barhoum, a spokesman for Hamas, which rules Gaza, told Al Jazeera that "this is a natural result for the Israeli aggression on our people". "If the Israel continues its aggression then all options are open."
The bus driver, who escaped largely unscathed, told reporters he had not seen anyone suspicious get on board. "I felt the explosion ... Smoke was everywhere, you couldn't see a thing," he said.
An Israeli who witnessed the explosion told Army Radio the bus was "completely charred". Other witnesses reported seeing a man leaving a bomb on the bus and then running off.
Celebratory gunfire rang out in Gaza City when local radio stations reported news of the Tel Aviv explosion. At least 139 Palestinians have been killed in Gaza since bombardment began on Wednesday. Five Israelis have been killed by rockets fired from Gaza.
The US condemned the bus bombing and said it would help Israel catch those responsible. "The United States will stand with our Israeli allies, and provide whatever assistance is necessary to identify and bring to justice the perpetrators of this attack," White House press secretary Jay Carney said in a statement.
The last time a bomb blast hit Tel Aviv was in April 2006, when a Palestinian suicide bomber killed 11 people at a sandwich stand near the old central bus station.
Gaza fighters have fired at least four rockets at the Mediterranean city over the past week, but they scored no direct hits and caused no casualties.
Israel launched its military offensive with the stated aim of halting all missile launches out of the Gaza Strip, which lies about 70km south of Tel Aviv.