In this image made from amateur video released by the Ugarit News and accessed Monday, July 23, 2012, Free Syrian Army soldiers pose for a photograph after destroying two military tanks during clashes with Syrian government troops in Aleppo, Syria. The Syrian regime acknowledged for the first time Monday that it possessed stockpiles of chemical and biological weapons and said it will only use them in case of a foreign attack and never internally against its own citizens. Aleppo, Syria's biggest city with about 3 million residents, has been the focus of rebel assaults by a newly formed alliance of opposition forces called the Brigade of Unification. (AP Photo/Ugarit News via AP video) TV OUT, THE ASSOCIATED PRESS CANNOT INDEPENDENTLY VERIFY THE CONTENT, DATE, LOCATION OR AUTHENTICITY OF THIS MATERIAL

BEIRUT - Fierce clashes flared for a fourth day Tuesday in Syria's commercial capital of Aleppo, showing the resilience of a rebel assault on regime targets.

President Barack Obama warned Syria against using its chemical weapons after Damascus acknowledged for the first time on Monday that they possessed such weapons and threatened to use them against any "foreign aggression."

The state news agency said regime forces fought with rebels in the Aleppo neighbourhoods of Salaheddine and Sukkari and claimed they had inflicted heavy losses. The Britain-based Syria Observatory, meanwhile, reported heavy fighting after midnight in several other neighbourhoods as well as shelling by regime forces.

Some areas also saw protests calling for the fall of the government early in the morning, the Observatory reported.

On Sunday, a newly formed alliance of rebel groups called the Brigade for Unification announced an operation to liberate Aleppo, the country's largest city with about three million people.

Reports from opposition activists indicate the fighting has been restricted to certain outer neighbourhoods and hasn't reached the city centre. Monday online video though showed rebels disabling at least two regime tanks.

Damascus, which saw an even fiercer rebel assault last week, appears to be largely in government hands once more as government troops scoured neighbourhoods for the remnants of rebel fighters, the state news agency reported.

Syria warned the international community Monday that it had chemical weapons and would use them in the case of any international aggression. The regime said it would not use them against its own people.

There had been fears that the embattled regime would use chemical weapons as a final desperate measure against the 17-month old rebellion. But the promise not to use them against Syrians was not entirely reassuring because officials have long characterized the rebels as foreign terrorists.

In a speech before a veterans' association in Reno, Nevada, Obama warned Syria against their use.

"Given the regime's stockpiles of chemical weapons, we will continue to make it clear to Assad and those around him that the world is watching, and that they will be held accountable by the international community and the United States, should they make the tragic mistake of using those weapons," he said.

Syria has become increasingly isolated on the international stage, with just regional ally Iran in its corner, as well as Russia and China protecting it from condemnation by the U.N. Security Council.

Most of its neighbours, however, have become increasingly hostile, including regional powerhouse Turkey. At a political rally late Monday, Turkish Prime Minister Recep Erdogan predicted the imminent end of the regime.

"This regime will go sooner or later. We believe that the people of Syria are ever closer to victory," he said.

Syrian rebel leaders operate on Turkish soil and arms for the opposition are believed to be entering the country from Turkey.

News from © The Associated Press, 2012