BEIRUT — 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.