Syrian opposition groups said their fighters were converging on Damascus amid growing questions about the government’s ability to keep control of the capital, where fierce clashes between security forces and rebels stretched into a third day on Tuesday.

Despite the overwhelming firepower of government troops, rebel forces appeared to hold their ground in several neighborhoods where the fighting was heaviest, according to members of the opposition.

The intense fighting in the capital marked the first time that many Damascus residents had seen overt signs of the bloody 16-month uprising against President Bashar al-Assad that has left at least 14,000 Syrians dead.

The rebels’ increasing boldness comes as the Syrian opposition appears to be gaining ground despite the government’s advantages in armament and training, military analysts said.

Independent analysts who have tracked the uprising say the rebels are staging more attacks nationwide and inflicting significantly more casualties than they were even a month ago. In June, the number of reported clashes hit a new high — 256, or an average of 8.5 per day — and the pace of fighting has surged since then, said Jeffrey White, a former analyst for the Defense Intelligence Agency. The figures are extrapolated from data provided by Syrian opposition committees and human rights organizations.

“It’s gone from intermittent clashes to sustained fighting in key provinces,” said White, a defense fellow for the Washington Institute for Near East Policy, a nonpartisan think tank.

Opposition fighters said they had shot down an army helicopter in the Damascus district of Qaboun, the Reuters news agency reported. The downing of the helicopter, if confirmed, may raise questions about whether rebel forces have received a shipment of antiaircraft weapons and other heavy arms they have been seeking for months from foreign supporters, including Qatar and Saudi Arabia.

Rebel forces are better equipped than they were even a few weeks ago, with an apparently plentiful supply of ammunition along with more machine guns and rocket-propelled grenades, White said. As a result, casualties among Syrian troops have soared over the past few weeks, to about 150 killed and wounded each day, he said. The rebel Free Syrian Army also is becoming more effective at destroying military vehicles and commandeering weapons and supplies from government forces and pro-government militias, he said.

Adding to the sense that some kind of showdown may be at hand, the Syrian government was moving troops from the Golan Heights toward Damascus and other trouble spots, Agence France-Presse reported.

The Britain-based Syrian Observatory for Human Rights said rebel forces are converging on Damascus to help with the fight. The deputy commander of the Free Syrian Army said in a telephone interview that the recent clashes marked the beginning of the group’s big push to take control of the city.

“Our strategy is to bleed down the regime forces and take over government buildings and key places in the capital,” Col. Malik Kurdi said. He denied that the opposition had been sent any heavy weapons.

With the government under extreme pressure, some observers have asked just how far Assad will go to defend his power. In an interview with the BBC on Monday, Nawaf al-Fares, a former Syrian ambassador to Iraq who defected last week, said that Assad is a “wounded wolf” who may resort to using chemical weapons.

The Wall Street Journal reported last week that Syria is moving its large arsenal of chemical weapons, which includes sarin and mustard gas. White House spokesman Josh Earnest declined to comment Tuesday on conjectures that Assad might use such weapons against Syrians but said, “What we have seen is inhumane brutality from the Assad regime, and that is something that has aroused the concern.”

Kurdi said he had heard reports of the army distributing chemical masks to troops.

U.S. officials and independent experts acknowledge that an end to the conflict is not in sight. Despite heavier losses and some high-profile defections, the regime has managed to keep its regular military divisions intact, particularly the elite units dominated by ethnic Alawites loyal to the Assad family, analysts said. The Alawites and other key ethnic and religious minorities appear to consider the conflict a struggle for their survival, having tied their fate to the success of Assad’s efforts to crush the rebellion through brute force.

“Once you’re committed to doing it that way, the only way to deal with resistance is to escalate,” said Anthony Cordesman, a defense expert at the Center for Strategic and International Studies. “There is no easy way out.”

Heavy shelling of residential areas of Damascus continued Tuesday as the army deployed armored vehicles and helicopters in an effort to root out the rebels. Opposition groups posted videos showing thick black smoke rising from Midan, one of the hardest-hit neighborhoods.

The state-run Syrian Arab News Agency reported that the army was fighting “armed terrorist groups” in several areas. An electricity converter station was attacked in Qaboun, according to the agency, which said security forces were trying to find the saboteurs. Another pro-Assad news site reported that the city’s deputy police chief had been killed in clashes with opposition fighters in Midan.

In Moscow on Tuesday, U.N.-Arab League special envoy Kofi Annan met with Russian President Vladimir Putin and said afterward that he hopes the members of the U.N. Security Council will agree to language on a Syria resolution.

Russia, along with China, has blocked U.N. action on the crisis. Russia fears a Western military intervention and argues that Assad still has significant support among Syrians and should not be forced out by external forces. It also warns that the crisis is providing an opening to Islamist extremists and has called for both sides to desist from violence.

Russian Foreign Minister Sergei Lavrov said later that it should be possible to reach consensus. “We are open to it,” he said, according to the Interfax news agency.

Lavrov had said Monday, before his meeting with Annan, that he thinks the Western powers are trying to “blackmail” Russia into agreeing to a resolution.

There was no comment in Moscow about the rebels’ claim that they had shot down a helicopter in Damascus — presumably a Russian-made aircraft. Syria has been Russia’s most reliable arms customer in the Middle East, and Russia recently completed the overhaul of a shipment of helicopters, which are being returned by sea to Syria.

“A helicopter was shooting with a machine gun on the rooftops,” one Damascus resident, who asked that her name not be used for security reasons, said in a direct message chat on Twitter. “I saw it with my own eyes. It was unbelievable.”

Another woman, a 30-year-old Damascus resident who also spoke on the condition of anonymity, said she saw two large buses carrying government troops crash Tuesday morning, leaving dazed and injured soldiers wandering across the al-Mezzeh highway.

“I feel that the city is going into chaos,” she said in a telephone conversation. “This is scaring me.”

At least 23 people were killed in the fighting in the city Tuesday, according to the Local Coordination Committees, an activist network.

Obama administration officials confirmed the increase in violence, particularly in the capital. State Department spokesman Patrick Ventrell said the fighting in Damascus was “the most sustained and widespread” in the city since the start of the uprising, and was accompanied by assaults by government troops on cities and villages throughout the country.

“Even in the face of this brutality, it’s clear that the regime can’t outlast these protests,” Ventrell told reporters Tuesday.

Warrick reported from Washington. Will Englund in Moscow and Suzan Haidamous and a special correspondent in Beirut contributed to this report.