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US warns hopes dim for Syrian settlement

BEIRUT — The United States has warned that the alleged use of chemical weapons by the forces of Syrian President Bashar Assad and the involvement of the Iranian-backed militant group Hezbollah in the civil war threaten to put a proposed political settlement out of reach.

Syrian troops pushed forward with their offensive against rebels Saturday, capturing Ahmadiyeh, a suburb near the Damascus international airport.

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The United States and Russia have been pressing for a peace conference to end Syria’s civil war in Geneva, but prospects for that have been dampened after regime battlefield victories and hardened positions by both sides. The death toll from the two-year war has surged to nearly 93,000.

President Obama’s decision to send lethal aid to Syrian rebels and the deepening involvement of trained Shi’ite fighters from Lebanon’s Hezbollah group also has raised the stakes, setting up a proxy fight between Iran and the West that threatens to engulf more of the Middle East.

Secretary of State John Kerry was quoted in a statement as saying the United States continues to work aggressively for a political solution with the goal of a second Geneva meeting. But ‘‘the use of chemical weapons and increasing involvement of Hezbollah demonstrates the regime’s lack of commitment to negotiations and threatens to put a political settlement out of reach,’’ he said in a phone call Friday with Foreign Minister Hoshyar Zebari of Iraq.

Egypt’s Islamist president said Saturday that he was cutting off diplomatic relations with Syria and closing Damascus’ embassy in Cairo, decisions made amid growing calls from hard-line Sunni clerics in Egypt and elsewhere to launch a ‘‘holy war’’ against Syria’s embattled regime.

Mohammed Morsi told thousands of supporters at a rally in Cairo that his government was also withdrawing the Egyptian charge d’affaires from Damascus. He called on Hezbollah to leave Syria, where the Iranian-backed Shi’ite militant group has been fighting alongside troops loyal to Assad against the mostly Sunni rebels.

The US reversal after months of saying the United States would not intervene in the conflict militarily came after Washington said it had conclusive evidence the Syrian regime had used chemical weapons, something Obama had said would be a ‘‘red line.’’

Syria has denied the accusations, saying Obama was lying about the evidence to justify his decision to arm the rebels.

Syria’s ally Russia also suggested Saturday that the evidence put forth by the United States of the use of chemical weapons does not meet stringent criteria for reliability.

Foreign Minister Sergey Lavrov of Russia said the US evidence does not include guarantees that it meets the requirements of the Organization for the Prohibition of Chemical Weapons.

He said the group specifies that samples taken from blood, urine, and clothing can be considered reliable evidence only if supervised by organization experts from the time they are taken up to delivery to a laboratory. The organization is the autonomous body for implementing the international Chemical Weapons Convention that went into effect in 1997.

Its website says Syria is one of six countries that have not signed or acceded to the convention. Lavrov, after meeting with his Italian counterpart, Emma Bonino, scoffed at suggestions that Assad’s regime would use chemical weapons in light of its apparent growing advantage against the rebels.

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