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Following its loss of the key border town of Qusair last week, Syria’s rebel leadership has made impassioned new pleas to the Obama administration for U.S. weapons shipments and described its situation as dire, according to senior administration officials.

The appeal came in telephone calls and meetings over the weekend, after rebel forces were routed by what they said was a combined force of pro-government Hezbollah, Iranian and Iraqi Shiite militias and Syrian air power. Those same forces are now said to be massing, along with government forces, for an attack on rebel-held areas in the strategic city of Aleppo.

What appeared to be a dramatic shift in the government’s favor, after a string of rebel victories earlier this year, overwhelmed talk of a peace conference sponsored by the United States and Russia that is supposed to take place in a matter of weeks.

Secretary of State John F. Kerry postponed a trip to the Middle East on Monday to attend a White House meeting of top national security aides on Syria on Wednesday.

The expansion of Syria’s civil conflict into a regional sectarian war, with Lebanon-based Hezbollah and Shiite allies in Iran and Iraq fighting alongside Syrian President Bashar al-Assad’s forces against the largely Sunni opposition, has long been a U.S. nightmare scenario.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.

But a senior administration official said it was unclear whether it would change the administration’s calculations on the provision of lethal weapons to the rebels.

“One can certainly make a better case that the situation is very bad for the United States, in addition to the FSA,” the official said, referring to the rebel Free Syrian Army, commanded by Gen. Salim Idriss. “That’s what’s changed.”

In a conversation with the State Department on Saturday, Idriss renewed his appeal for a no-fly zone over rebel-held areas of Syria, including the key northern supply corridor from Turkey to Aleppo.

His most urgent plea, however, was for arms and ammunition, including armor-piercing weapons and surface- to-air missiles, according to officials who are familiar with the conversation but spoke on the condition of anonymity about internal deliberations. “He said he was desperate,” an official said of Idriss.

The Pentagon has cautioned about the difficulties of establishing a no-fly zone, and the administration has resisted past appeals for weapons on grounds that Persian Gulf nations have been supplying sufficient quantities to the rebels. More-sophisticated arms, including surface-to-air missiles, could end up in the hands of Islamist extremists fighting alongside opposition forces, officials have said.

Russia and Iran are Assad’s principal arms suppliers. Although Kerry and his Russian counterpart, Sergei Lavrov, announced in early May that they would jointly sponsor a peace conference, plans for the meeting remain on hold.

White House press secretary Jay Carney said Monday that administration deliberations about peace talks and about aid to the opposition were “related, but we’re reviewing them separately.”

Kerry had been expected to go to Israel and the West Bank this week for the latest in a series of shuttle diplomacy visits aimed at restarting peace talks between the Israelis and Palestinians. State Department spokeswoman Jen Psaki confirmed Monday that Kerry scrapped the trip to attend what she called routine meetings on Syria and other subjects in Washington. Widening U.S. military support for the rebels is among the options being discussed, Psaki said.

“A wider range of options has been prepared for the president’s consideration, and internal meetings to discuss the situation” are ongoing, Psaki said. She would not address any recommendations or likely outcomes.

“As we’ve heard firsthand from General Idriss over the weekend, conditions on the ground have worsened, and that is greatly concerning,” Psaki said. “The bloodshed and the loss of innocent lives has grown worse. The increase of foreign fighters has led to a greater concern about sectarian violence.”

All options remain open, short of placing U.S. military forces on the ground in Syria, both Psaki and Carney said.

The lack of progress toward Israel-Palestinian peace talks has given Kerry less reason than he had hoped for a quick return to that issue. Palestinian President Mahmoud Abbas has not agreed to return to the table and is seeking U.S.-backed assurances or incentives from Israel to do so, said U.S., Israeli and Arab officials. Abbas has reportedly asked for the release of more than 100 Palestinian prisoners from Israeli jails.

The Palestinians had set an unofficial deadline of early June for Kerry to show progress, but have now extended the window until later this month, two officials said. All officials spoke on the condition of anonymity to discuss Kerry’s closed-door efforts.

A senior Israeli official said Israel remains committed to talks without preconditions and was “looking at ways in which we can convince” Abbas “to drop his preconditions.”

Kerry risks looking naive or weak if the standoff continues too long. U.S. diplomats were divided about whether Kerry should apply pressure in person this week, something he said last week he might have to do.