Israeli authorities and settlers have seized large tracts of land in the occupied West Bank for security zones around Jewish settlements beyond an Israeli-built barrier, a human rights group said on Thursday.

In a new report, the Israeli B'Tselem group said some 12 settlements east of the barrier had been fenced off under an official "Special Security Area" (SSA) plan, blocking Palestinian farmers from reaching their fields.

B'Tselem, which opposes Israeli settlement on territory occupied in the 1967 Middle East war, estimated that the overall area of some settlements in the plan had more than doubled.

Some 70,000 settlers live beyond the barrier of barbed wire-tipped fences and cement walls Israel is constructing in the West Bank, and security authorities view their settlements as particularly vulnerable to attack.

Asked about the B'Tselem report, the Israeli army said it had established security zones around settlements after they had been attacked repeatedly by Palestinians and dozens of Israeli civilians had been killed.

"The use of these zones has been approved a number of times by the Israeli Supreme Court. Any building in these zones is illegal," the army said.

B'Tselem said it could give only a rough estimate of the total territory closed to Palestinians but that at least 1,126 acres had been "unofficially annexed" outside the 12 settlements.

It said half of the closed-off land was privately owned by Palestinians.

Control of the land, the report said, was achieved by fencing it off or through attacks by settlers and sometimes soldiers against Palestinians who ventured near it.

A spokesman for the Yesha settler's council, an umbrella group for Israelis living in the West Bank, said taking control of the areas was a necessary security measure.

"It must be clear to B'Tselem that if their demands are heeded, it will be easier to murder Jews," Yishai Hollander said.

Israeli Vice Premier Haim Ramon has proposed offering compensation to settlers living beyond the West Bank barrier who agree to move to Israel or to major settlement enclaves it intends to keep in any peace deal with the Palestinians.

Israel says the barrier, deemed illegal by the International Court of Justice because it is being built on occupied territory, keeps Palestinian suicide bombers out of its cities.

The Palestinians call the project a land grab and say settlement expansion could deny them a viable and contiguous state in the West Bank.