TEL AVIV (Reuters) - A bomb exploded on a bus in central Tel Aviv on Wednesday, wounding 15 people in what Israeli officials said was a terrorist attack that could complicate efforts to secure a ceasefire in the Gaza Strip.
Celebratory gunfire rang out across Gaza as the news spread and the territory's Islamist rulers Hamas praised the bombing, but no one claimed immediate responsibility.
The blast shattered windows on the bus as it drove along a tree-lined street next to Israel's huge defence ministry headquarters. Israel's ambulance service said four people suffered moderate-to-severe injuries and 11 were lightly hurt.
Police said it was not a suicide attack and suggested that someone might have left the device on the number 142 bus.
The 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. The blue and white vehicle was not torn apart by the blast, indicating it was a relatively small device.
The bombing happened on the eighth day of an Israeli offensive against the Gaza Strip and coincided with intensive diplomatic efforts to secure a lasting truce.
Hamas spokesman Sami Abu Zuhri hailed the explosion.
"Hamas blesses the attack in Tel Aviv and sees it as a natural response to the Israeli massacres...in Gaza," he told Reuters. "Palestinian factions will resort to all means in order to protect our Palestinian civilians in the absence of a world effort to stop the Israeli aggression."
Sweet cakes were handed out in celebration in Gaza's main hospital, which has been inundated with wounded from the round-the-clock Israeli bombing and shelling.
"You opened the gates of hell on yourselves," Hamas's armed wing, the al-Qassam brigades, said on Twitter. "Oh Zionists, you have to drag yourselves out of hell, go back home now, go back to Germany, Poland, Russia, America and anywhere else."
The last time a bomb blast hit Israel's commercial capital was in April 2006, when a Palestinian suicide bomber killed 11 people at a sandwich stand near the old central bus station.
Hamas militants have fired at least four rockets at the laid-back Mediterranean metropolis over the past week, but they scored no direct hits and caused no casualties.
Israel launched its air offensive with the stated aim of halting all missile launches out of the Gaza Strip, which lies some 70 km (40 miles) south of Tel Aviv, a cosmopolitan city renowned for its nightlife and vibrant beach culture.
Hamas had warned when the latest conflict flared that it would not confine itself to unleashing rockets.
"This was a terror attack. There is a massive preparedness within the police and security forces. We must keep awareness to a maximum. These are not normal times," said Israel's Police Chief Yohanan Danino.
U.N. Secretary-General Ban Ki-moon denounced the attack, saying nothing justified the targeting of civilians.
The United States, Israel's main ally, also condemned the bus bombing. "These attacks against innocent Israeli civilians are outrageous," the White House said.
More than 140 Palestinians, more than half of them civilians, have died so far in Israel's Gaza offensive. Five Israelis, including one soldier, have also been killed.
U.S. Secretary of State Hillary Clinton, trying to calm tensions over Gaza, flew from Israel to Cairo to meet Egyptian President Mohamed Mursi, who is spearheading ceasefire negotiations.
Reporting by Jeffrey Heller, Ari Rabinovitch and Nidal al-Mughrabi; Writing by Crispian Balmer; Editing by Alistair Lyon