UNITED NATIONS — The Palestinians accused Israel on Monday of systematically destroying prospects for a two-state solution to their decades-long conflict with its continuing campaign of settlement building.

Palestinian U.N. observer Riyad Mansour sent a protest letter to the U.N. secretary-general, Security Council and General Assembly two days before the Quartet of Mideast mediators — the U.S., U.N., European Union and Russia — meets in Washington to discuss the long-stalled peace process.

Mansour said Israel’s “illegal and destructive plans” to build new settlements underscore “the dubious nature” of its claims of readiness to negotiate a peace deal.

Israeli-Palestinian talks remain frozen over Palestinian demands that Israel stop building on lands they claim, and agree to negotiate borders based on lines Israel held before capturing the West Bank, Gaza and east Jerusalem in 1967.

Israel rejects those conditions and has defied international pressure to freeze settlement construction. Israeli Prime Minister Benjamin Netanyahu insists that negotiations resume without what he has portrayed as preconditions.

“The main obstacle to peace is not settlements,” Karean Peretz, spokeswoman for Israel’s U.N. Mission, said Monday. “It is the so-called ‘claim of return’ and the Palestinians leadership’s refusal to return to the negotiating table. We need direct negotiations, not more provocations at the U.N.”

The Palestinians have long demanded that Palestinian refugees and their descendants, who now number in the millions, be allowed to return to Israel. The Israeli government insists that the country must be recognized as the Jewish homeland, which would require the Palestinians to accept that most refugees will be denied the “right of return” to what is now Israel.

The Quartet has called for a resumption of Israeli-Palestinian negotiations with the goal of a peace agreement by the end of 2012. But in late February, U.N. political chief B. Lynn Pascoe told the Security Council that exploratory low-level talks in Amman, Jordan between the Israelis and Palestinians had stalled and prospects for resuming direct negotiations “remain dim.”

Nonetheless, the Quartet on March 12 again urged the Israelis and Palestinians to return to negotiations and reach an agreement no later than the end of the year.

The Quartet’s top representatives — U.S. Secretary of State Hillary Clinton, U.N. Secretary-General Ban Ki-moon, European Union foreign policy chief Catherine Ashton and Russian Foreign Minister Sergey Lavrov — are scheduled to discuss the latest Mideast situation at Blair House in Washington on Wednesday morning on the sidelines of a meeting of foreign ministers of the eight major industrialized nations.

In his letter, Mansour said Israel “continues its systematic destruction of the two-state solution with its continuing illegal settlement campaign” which he said is “being deliberately waged in an attempt to seize more Palestinian land and entrench its control over the Palestinian territory.”

Mansour appealed to the international community and especially to the Security Council “to condemn Israel’s illegal settlement activities” and take urgent measures to pressure Israel to immediately halt new construction.