JERUSALEM (AFP) - Israeli troops arrested some 30 Palestinians in the West Bank overnight as they ramped up a search for three teenagers believed kidnapped by Hamas, the army said on Thursday.

The arrests raised the number of Palestinians detained since the youths went missing a week ago to 280, around three-quarters of them members of the Islamist Hamas movement, a statement said.

The teenagers, two of them minors, disappeared from a popular hitchhiking spot in the sprawling southern West Bank Gush Etzion settlement bloc late on June 12.

Israel accuses Hamas of being behind the abductions and has launched a far-reaching military operation aimed at finding the teenagers and also crushing the movement's infrastructure in the West Bank.

"They were kidnapped by Hamas, we had no doubt of that. It's absolutely certain," Prime Minister Benjamin Netanyahu said in a Thursday briefing at a West Bank army base near Hebron.

Mr Netanyahu did not offer evidence, but said "we know more today that we did a few days ago", implying the operation could take much more time.

"I expect (Palestinian) president (Mahmud) Abbas to dissolve the union with this murderous terror organisation. I think that's important for our common future," he added.

On June 2, Mr Abbas appointed a Palestinian unity government made up of independents, which is committed to renouncing violence, but is backed by Hamas which is sworn to the Jewish state's destruction.

Soldiers had overnight searched around 100 locations and stormed 10 Hamas organisations, which the army said were used by the Islamists "to recruit, disseminate information and enable cash flow".

One of the sites targeted was the students' union at Bir Zeit University near Ramallah, Palestinian sources said.

At a meeting with the families of the missing youths, President Shimon Peres said Israel should pursue its crackdown on Hamas.

"We must continue to pressure the terrorists, to remove their sources of funding," he told them in remarks relayed by his office. "Just as there were sanctions against Iran, we must raise the sanctions against any source of terror."

Defence Minister Moshe Yaalon meanwhile issued an order outlawing activities of a Britain-based Muslim charity over what he said were its ties with Hamas.

"The IRW (Islamic Relief Worldwide) is one of the sources of Hamas's funding," a statement from his office said, indicating that some of IRW's West Bank and Gaza offices were "managed by Hamas members".

The group will also be prohibited from transferring money to the West Bank.

Mr Yaalon said the pressure on Hamas was aimed at its "leaders and activists, as well as its civilian infrastructure".

The widespread search and arrest operations sparked clashes overnight in the northern cities of Jenin and Nablus, with Palestinians throwing petrol bombs and firing at the soldiers, the army said.

The Palestinian presidency warned on Thursday of "ongoing collective punishment measures by Israel in all Palestinian territories, especially besieged Hebron", affecting "hundreds of thousands of innocent residents".