The buses exploded into flames just seconds apart and about 100 yards away from each other in the center of Beersheba - the deadliest suicide strike in nearly a year.

Israel had attributed the lull to its separation barrier, arrest sweeps and widespread network of informers.

The Hamas militant group claimed responsibility, issuing a leaflet in Hebron - the closest Palestinian city to Beersheba - saying it was avenging Israel's assassinations of two of its leaders earlier this year.

Israeli Prime Minister Ariel Sharon met with his security advisers to plan a response, expected to include a military operation in Hebron. Just hours before the attack, Sharon presented his hardline Likud Party with the most detailed timetable yet for Israel's withdrawal from the Gaza Strip.

Despite the bombings, Sharon promised to push forward with the Gaza pullout, while insisting Israel would keep fighting terrorism ''with all its might.''

''This [the attack] has no connection to disengagement,'' he said, referring to his program to separate Israel from the Palestinians.

In Washington, Secretary of State Colin Powell condemned the bombings and offered condolences to Israeli Foreign Minister Silvan Shalom, and State Department Spokesman Richard Boucher said Hamas must be put out of business.

White House spokesman Scott McClellan said President Bush had been briefed. ''There's simply no justification for the killing of innocent civilians,'' McClellan said.

Israeli officials said the bombings proved the need for the barrier now under construction between Israel and the West Bank. The section between Hebron and Beersheba has not been built.

''We should go ahead speedily now and finish construction of this fence,'' government spokesman Avi Pazner told The Associated Press.

The barrier, which Israel says is necessary to keep out suicide bombers, has been widely condemned internationally because of the hardships it imposes on Palestinians.

Palestinian analyst Hani al-Masri agreed with the Israeli assessment of the lull in suicide attacks.

He told the AP that it resulted from Israel's assassination of Hamas leaders and the difficulties of infiltrating posed by the barrier.

''But now, the military operations [attacks] are a way for Hamas to increase its popularity among Palestinians,'' he said, noting that until Tuesday, the Islamic group had not carried out its promise to avenge Israel's killing of its founder and his successor.

It had been nearly six months since Israelis last experienced the scene of charred buses, mangled bodies and screaming sirens that played out in Beersheba on Tuesday.

The last suicide bombing in Israel took place on March 14, when 11 people were killed at the port of Ashdod.

Tuesday's attack was the deadliest since a female suicide bomber killed 21 people nearly a year ago in the northern city of Haifa - an attack that prompted Israel to assassinate Sheik Ahmed Yassin, the spiritual leader of Hamas, and his successor, Abdel Aziz Rantisi.

Israel's rescue service said 30 of the wounded in Tuesday's attack were in serious condition. Police said the death toll of 16 did not include the bombers.