Washington (AFP) - The United States rebuked Israel on Wednesday over what it said was its accelerated building of Jewish settlements in the occupied West Bank in the face of mounting international concern.

"This significant expansion of the settlement activity poses a serious and growing threat to the viability of a two state solution," President Barack Obama's spokesman Josh Earnest said.

"We are particularly troubled by the policy of retroactively approving illegal outposts and unauthorized settlements," he added, briefing reporters aboard Air Force One en route to China.

Washington has long been concerned that Israel's building on occupied Palestinian land is undermining hopes for a Middle East peace deal, but Wednesday's warning was unusually strongly worded.

Officials said Israel's actions ran counter to the advice laid out by the Middle East Quartet, a contact group comprising the United Nations, European Union, Russia and the United States.

"We are deeply concerned by the government's announcement to advance plans for these settlement units in the West Bank," State Department spokesman John Kirby told reporters.

"Since the Quartet report came out we have seen a very significant acceleration of Israeli settlement activity that runs directly counter to the conclusions of the report," he complained.

"So far this year Israel has promoted plans for over 2,500 units including over 700 units retroactively approved in the West Bank.

"These policies have effectively given the Israeli government's green light for the pervasive advancement of settlement activity in a new and potentially unlimited way," Kirby warned.

"This significant expansion of the settlement enterprise poses a very serious and growing threat to the viability of the two-state solution," he added, implying Israel could torpedo peace hopes.

- 'Perpetual occupation' -

The United States has urged Israel and the Palestinians to "take meaningful steps" to build trust, but Israel has instead promoted the building of thousands of new settlement homes.

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A senior US official said the building boom "fundamentally undermines the prospects for a two-state solution and risks entrenching a one-state reality of perpetual occupation and conflict."

Another US official confirmed that the choice of language represented an American decision to toughen its tone with Israel.

Israel has defended its settlement building, arguing that Jews have lived in the West Bank and Jerusalem -- the divided city claimed by both Israel and Palestinians as a capital -- for thousands of years.

But the senior US official warned the Israeli government risks undermining its own case with its aggressive settlement program and a "dramatic acceleration" in its demolition of Palestinian buildings.