Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

Syria’s rebels on Friday criticized the U.S. decision to offer small-scale military assistance as late and inadequate, saying they will need heavy weapons to counter the growing challenge posed by a reinvigorated Syrian army that is already receiving foreign help.

But the real significance of the policy shift may lie in the signal it sends to the increasingly polarized region that America does not intend to remain on the sidelines and allow Syrian President Bashar al-Assad to prevail over the outgunned rebels.

The fact that the United States is now committing itself militarily to the rebels at a time when Russia, Iran and Lebanon’s Hezbollah movement are escalating their support for Assad draws the United States inexorably into what is rapidly becoming a global proxy war for control of Syria, analysts said.

“The general direction of travel is toward greater Western involvement,” said Salman Shaikh, director of the Brookings Doha Center in Qatar.

The White House on Thursday said that the United States will for the first time send direct military assistance to the Syrian rebels, following the conclusion by U.S. intelligence agencies that the Syrian government had used small quantities of chemical weapons in its efforts to defeat the armed rebellion.

The Obama administration announced Thursday it would arm Syrian rebels. Post columnist David Ignatius explains how and why the step is being taken. (The Fold/The Washington Post)

It also followed months of battlefield setbacks for those seeking Assad’s ouster, culminating last week in the loss of the strategic western town of Qusair near Lebanon’s border to a force in which Lebanon’s Shiite Hezbollah movement played a leading role.

On Friday, government forces launched their heaviest attack in months against rebel-held areas in the divided northern city of Aleppo, the country’s commercial capital, pounding rebel positions with artillery and attempting to break through rebel lines in the frontline neighborhood of Sakhour.

The push fell short of the full-scale offensive to retake the city that has been widely touted as imminent. Rebel leaders say they are confident they can withstand a government assault in the north, where the regime’s supply lines are stretched and the rebels have access to the Turkish border, a key source of weapons supplies, whether from the black market or the Persian Gulf countries that have been supporting them. It would also be one of the likely routes for any U.S. weapons supplies.

“On the Aleppo front we are the most powerful and we are putting the regime and Hezbollah under pressure,” said Col. Abdul Jabbar Akaidi, head of the military council in Aleppo, who returned to the city Thursday from Qusair after the rebel rout.

But the loss of Qusair and the threat of a government assault in Aleppo underscored a growing sense of desperation among the rebels that they are being forced onto the defensive after nearly a year of battlefield gains that saw them seize control of large swaths of territory in the north and east of the country.

Instead, the regime is steadily gaining ground in the crucial battle for control of the suburbs ringing Damascus and is routing the rebels in the central province of Homs, putting Assad in a strong position to retain control of the capital.

Louay al Mokdad, political and media coordinator for the umbrella Free Syrian Army, said he welcomed the White House decision, but called it a “late step.”

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

“If they send small arms, how can small arms make a difference?” he asked. “They should help us with real weapons, antitank and antiaircraft, and with armored vehicles, training and a no-fly zone.”

Options for military aid

U.S. officials are expected to meet with Gen. Salim Idriss, head of the rebel Supreme Military Council, over the next two days to discuss details of military assistance that Washington will provide. The White House has not specified what the assistance may entail, and that may be deliberate, said Andrew J. Tabler of the Washington Institute of Near East Affairs.

“I think they are being tactically ambiguous, for their own reasons and to make Assad sweat,” he said.

Some rebel leaders expressed doubt that any meaningful support would actually arrive after months of statements from the U.S. government promising nonlethal aid that they say hasn’t materialized.

“We have honestly lost hope,” said Mosab Abu Qutada, a spokesman for the rebel military council in Damascus. “We were promised a lot before, and they never kept their promises.”

U.S. officials have ruled out sending ground troops and on Friday played down the likelihood of a no-fly zone in Syria, calling it “difficult” and “dangerous.” But a military exercise currently under way in Jordan points to a growing level of preparedness by the United States and its allies for a wide range of options.

Around 5,000 U.S. troops are among 8,000 from 19 nations taking part in the Eager Lion exercise, which also includes F-16 and F-18 fighter jets and a battery of Patriot missiles that will remain behind in Jordan after the drill concludes next week.

A Jordanian government official dismissed as “premature” reports that there are plans for Jordan to serve as the base for a future Syrian no-fly zone. The kingdom currently has no plans to “be part of any international military action against Syria,” he said.

However, a Jordanian military official said that Amman and Washington drew up plans for such a zone in March and that the dispatch of the missiles and fighter jets represented a “first phase.”

“We already know what a no-fly zone over southern Syria will look like, how to enforce it and who we will work with on the ground,” said the official, who spoke on the condition of anonymity because of the sensitivity of the subject. “All we are waiting for is the final decision.”

Sectarian overtones

The conflict is also taking on increasingly sectarian overtones, with the growing entanglement of the Shiite Hezbollah movement inflaming Sunni anger across the region. Hours before the White House announcement that it would send unspecified “military assistance” to the rebels, a group of senior Sunni clerics meeting in Cairo called for “jihad” in Syria.

In a defiant speech delivered to supporters Friday night, Hezbollah leader Hasan Nasrallah blamed the regime’s opponents for seeking to turn the conflict into a sectarian one, and said Hezbollah would not be deterred in its support for the regime.

“The world is coming to fight in Syria . . . with its money, with its media, and even with this lie that they will start to arm the opposition. The opposition has been armed a long while ago.”

He did not address directly reports that Hezbollah fighters have been deployed as far afield as the northern city of Aleppo and the southwestern city of Daraa, but he made it clear Hezbollah is prepared to fight “wherever needed.”

“We will be where we need to be. What we started to carry out, we will continue,” he said.

Taylor Luck in Amman and Ahmed Ramadan in Beirut contributed to this report.