The United States said on Saturday it would keep F-16 fighters and Patriot missiles in Jordan at Amman's request, and Russia bristled at the possibility they could be used to enforce a no-fly zone inside Syria.
Washington, which has long called for President Bashar al-Assad to step down, pledged military support to Syrian rebels this week, citing what it said was the Syrian military's use of chemical weapons - an allegation Damascus has denied.
Defense Secretary Chuck Hagel has approved a Jordanian request for American F-16s and Patriot missiles to remain in the Western-backed kingdom after a joint military exercise there next week, a Pentagon spokesman said.
Western diplomats said on Friday Washington was considering a limited no-fly zone over parts of Syria, but the White House noted later that it would be far harder and costlier to set one up there than it was in Libya, saying the United States had no national interest in pursuing that option.
Russia, an ally of Damascus and fierce opponent of outside military intervention in Syria, said any attempt to impose a no-fly zone using F-16s and Patriots from Jordan would be illegal.
"You don't have to be a great expert to understand that this will violate international law," Foreign Minister Sergei Lavrov said.
The idea of a no-fly zone was endorsed by Egypt, the biggest Arab nation. President Mohamed Mursi, an Islamist more distant from Washington than his deposed military predecessor, made a keynote speech in Cairo throwing Egypt's substantial weight more firmly than before against President Bashar al-Assad.
Despite their differences, the United States and Russia announced in May they would try to convene peace talks involving the Syrian government and its opponents, but have set no date.
U.S. Secretary of State John Kerry said chemical attacks by Syrian forces and Hezbollah's involvement on Assad's side showed a lack of commitment to negotiations and threatened to "put a political settlement out of reach".
Kerry had not previously expressed such pessimism about prospects for the conference, which has run into many obstacles.
These include disarray in the Syrian opposition and military gains by the Syrian army and its Lebanese Hezbollah allies against rebels who have few ways to counter Assad's air power.
The involvement of Hezbollah fighters on the side of Assad, a fellow ally of the main Shi'ite power Iran, has galvanized Arab governments, including Egypt, behind the rebels, who mostly follow the Sunni version of Islam that dominates the Arab world.
That has hardened sectarian confrontation across the region, which some Arabs hope might be softened by the election of the moderate Hassan Rohani as Iran's president - though few believe he can truly influence Tehran's supreme leader.
Mursi, addressing thousands of cheering supporters at a stadium gathering organized by Egyptian Sunni clerics, demanded Hezbollah pull out of Syria and, after his Muslim Brotherhood joined calls for jihad against Assad and his Shi'ite allies, the president said Cairo had now cut diplomatic ties with Damascus.
Egypt's powerful, U.S.-backed army seems unlikely to involve itself in Syria, but religious passions are running high and more Egyptian volunteers could travel to join the rebels.
The pro-opposition Syrian Observatory for Human Rights said Syrian jets and artillery had again attacked Jobar, a battered district where rebels operate on the edge of central Damascus.
It said heavy artillery was also shelling opposition fighters in the provinces of Homs, Aleppo and Deir al-Zor.
Western powers have been reluctant in the past to arm Syrian insurgents, let alone give them sophisticated anti-aircraft missiles that might fall into the hands of Sunni Islamist insurgents in rebel ranks who have pledged loyalty to al Qaeda.
Free Syrian Army (FSA) commander Salim Idriss told Reuters late on Friday that rebels urgently needed anti-aircraft and anti-tank missiles, as well as a protective no-fly zone.
"But our friends in the United States haven't told us yet that they are going to support us with weapons and ammunition," he said after meeting U.S. and European officials in Turkey.
A source in the Middle East familiar with U.S. dealings with the rebels has said planned arms supplies would include automatic weapons, light mortars and rocket-propelled grenades.
The United Nations says at least 93,000 people, including civilians and combatants, have died in the Syrian civil war, with the monthly death toll averaging 5,000 in the past year.
Abu Nidal, from the Islamist Ahrar al-Sham rebel group, said U.S. help was welcome, but questioned how effective it would be.
"I doubt the influx of weapons will significantly tip the balance into our favor," he said via Skype. "They might help push back regime offensives of the last few days."
SYRIAN OFFICERS DEFECT
Abu Nidal's faction is not part of the more moderate FSA, Washington's chosen channel for military aid, but he said the two groups fight alongside each other on the battlefield.
The FSA was set up by defectors from the Syrian military in August 2011, but many rebel factions operate independently.
Assad's armed forces have remained relatively cohesive, although a Turkish official said 71 Syrian army officers, including six generals, had just defected to Turkey, in the biggest such mass desertion in months.
Western nations have stopped short of arming Syrian rebels or mounting an air campaign as they did, with U.N. approval, to help Libyan insurgents topple Muammar Gaddafi in 2011.
Intervening against Assad is considered riskier because Syria has a stronger military, sits on the sectarian faultlines of the Middle East, and is supported by Iran and Russia, which has vetoed three U.N. Security Council resolutions on Syria.
Yet an apparent shift in the military balance in Assad's favor, especially with the arrival of thousands of Shi'ite fighters from the Iranian-backed Hezbollah group, has made his swift removal look unlikely without outside intervention.
However, Israel's defense minister suggested the pendulum could still swing the other way, despite the capture this month of Qusair, a former rebel stronghold near the Lebanese border.
"Bashar al-Assad's victory in Qusair was not a turning point in the Syrian civil war, and I do not believe that he has the momentum to win," said Moshe Yaalon, who is visiting Washington.
"He controls just 40 percent of the territory in Syria. Hezbollah is involved in the fighting in Syria and has suffered many casualties in the battles, and as far as we know, it is more than 1,000 casualties," Yaalon said in a statement.
"We should be prepared for a long civil war with ups and downs."
Israel has not taken sides in Syria, but does not want to see any Western anti-aircraft missiles or other advanced arms reach Islamist militants hostile to the Jewish state.