Widening its five-day campaign against Palestinian militants, Israel for the first time fired artillery shells into the Gaza Strip on Wednesday and shut down 15 West Bank offices suspected of distributing money to families of bombers from the Hamas (search) and Islamic Jihad (search) groups.

Israeli aircraft also fired missiles at several Gaza targets, knocking out power in Gaza City for most of the night, damaging several buildings and destroying an overpass, but there were no injuries. In the West Bank, Israel rounded up 24 suspected militants, bringing the number of people arrested since the weekend to more than 400.

A senior Israeli army commander did not rule out shelling Gaza towns.

"We will warn the population, make sure that they leave the area, while we target the sources of rocket fire," said Maj. Gen. Israel Ziv, the army chief of operations.

In other developments, a meeting between Israeli Prime Minister Ariel Sharon and Palestinian leader Mahmoud Abbas (search), tentatively set for Sunday, was postponed, apparently because of the flare-up.

Also, a Sharon adviser and an Israeli Cabinet minister said Wednesday that Israel could withdraw from some parts of the West Bank and annex others to its territory if peace efforts remain bogged down.

The comments by adviser Eyal Arad and Cabinet Minister Tzahi Hanegbi on Israel Army Radio marked the first time Sharon confidants talked in public about the idea of additional unilateral moves by Israel after the Gaza pullout in September. Sharon's office said Israel remains committed to a negotiated deal with the Palestinians.

The Israeli strikes were triggered by weekend rocket fire from Gaza at Israeli border towns. Since then, Hamas and Islamic Jihad have announced they would halt attacks, but Israel said it would press ahead with the campaign, including targeted killings of militant leaders.

On Tuesday, Israeli Defense Minister Shaul Mofaz threatened Hamas leaders Mahmoud Zahar and Ismail Hanieh by name, saying they could be the next targets.

Israel says the strong reaction is necessary to show that new rules are in place following its withdrawal from Gaza after a 38-year occupation and that attacks from the area will not be tolerated.

"Terrorism must be rooted out," Vice Premier Shimon Peres told Israel Radio on Wednesday.

However, Israel also appears to be seizing an opportunity to deliver a major blow to Hamas, which had been largely off-limits after it agreed in February to abide by an informal truce. Israel has repeatedly demanded that Abbas disarm Hamas, but Abbas has chosen to co-opt the militants instead.

Hamas commentator Ghazi Hamad said that "Israel could weaken Hamas, but not destroy it." The group, which opposes the existence of Israel, has killed hundreds of Israelis in attacks since violence resumed.

"Hamas now is a political movement, it's a culture, with individuals, institutions, schools, universities and a huge military wing," said Hamad, editor-in-chief of the Hamas weekly, The Message, which is published in Gaza.

Hamas charged that Israel also is trying to weaken the group politically before Palestinian parliamentary elections in January. Among those rounded up in recent days were dozens of Hamas candidates for parliament and local councils.

Hamas is expected to make a strong showing in the parliament vote and in a new round of local elections — the third of four — to be held in more than 100 towns and villages Thursday.

Abbas, in Egypt seeking help to halt the rapidly deteriorating conditions in Gaza and the West Bank, said Hamas was partly to blame for the latest violence.

He said the militant group aggravated the situation in Gaza when it insisted on celebrating the Israeli pullout Friday and later refused to admit that an explosion during that celebration was caused by one of the group's cars and not by Israel.

"Brothers in Hamas refused this talk and started firing missiles on the Israelis that made them say they did not have a partner in the peace process and started random attacks and bombing that destroyed many areas," Abbas told Arab League representatives.

Abbas said Hamas has the right to retaliate, but added: "We agreed in Cairo on a collective truce and the retaliation, therefore, should be collective, too, because this is an Israeli carnage, the result of which must be shared by all parties."

In a new phase of Israel's offensive, troops closed down 15 offices, including charities linked to local mosques, across the West Bank, the military said. Israel said money for the families of suicide bombers and militants jailed by Israel was distributed through these offices.

Hamad said Hamas has a support network extending from East Asia through the Gulf to Europe, and that Israel would not be able to stop the flow of money and donations.

"Israel is monitoring, but it can't monitor every channel," he said, suggesting that some of the money is brought in through tunnels under the Gaza-Egypt border.

The military activity came after various Palestinian factions, including Hamas and the ruling Fatah movement, renewed their commitment to a cease-fire, although they also said they reserve the right to retaliate for perceived Israeli truce violations.

Israeli army spokeswoman Capt. Yael Hartmann called the militants' pledges "meaningless" and said the open-ended military operation would continue.

"The ... operations to destroy the infrastructure are not based on Hamas' declarations," she said. "As of now, we're continuing with our operational plans."

Mofaz pledged Tuesday to step up pressure on the militants, saying a ground invasion into Gaza is possible as a last resort. He spoke while touring an Israeli artillery battery near Gaza.

"This battery ... is not meant to be decoration. It is operational, within range and it will respond against every firing of a Qassam in real time, and that is to deter," he said.

Later, troops fired live artillery shells into northern Gaza for the first time, hitting an open area that the army said was used to fire rockets. In recent days, gunners fired dummy shells, in part to calibrate the artillery.