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.

The U.S. and Russia, meanwhile, both warned Syrian officials 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.

Also on Tuesday, Syrian President Bashar al-Assad appointed several new intelligence officials, including a new head of national security — replacing those who were killed last week in a bomb attack in Damascus.

Ali Mamlouk replaces Hisham Bekhtyar as head of Syria's Internal Security Directorate. Al-Assad earlier replaced the country's defence minister, who was also killed in Wednesday's suicide bombing, with Gen. Fahad Jassim al-Freij.

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 neighborhoods for the remnants of rebel fighters, the state news agency reported.

Chemical weapon concerns

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 UN Security Council.

'This regime will go sooner or later'—Turkish Prime Minister Recep Erdogan

But even Russia has chided Syria over its threatened use of chemical weapons, reminding Damascus on Tuesday of its pledge to never use them, and appearing to show a degree of irritation with its longtime ally.

Closer to home, Syria's neighbours 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.

Israel is also keeping a close eye on its neighbour and longtime enemy.

The Jewish state is concerned those weapons could wind up with extremist groups allied with the Syrian rebels, or in Lebanon with Hezbollah, according to Irris Makler, a CBC freelance reporter based in Jerusalem.

"[Israel says] if that happens, they will take military action," said Makler.

According to the United Nations Refugee Agency (UNHCR), as many as one million people have been internally displaced since the Syrian conflict started.

Thousands more have fled Syria and headed to neighbouring countries.

"In Lebanon, the UNHCR says there are 30,000 officially registered refugees fleeing the violence in Syria," CBC's Margaret Evans said Tuesday.

But Evans noted that in recent days, there has been a surge in people crossing over from Syria to Lebanon, "so there's a question of how many are not registered."

Some are fleeing the fighting "with no more than their shoes on," she said, reporting from a border crossing between Syria and Lebanon. Others say they are simply taking a holiday, she added.

The UNHCR said over the weekend that it is working with partners to try to verify numbers and assess the needs of the people who recently arrived in Syria from Lebanon.

According to a UNHCR spokesman, the total number of registered Syrian refugees in countries such as Jordan, Iraq, Lebanon and Turkey has nearly reached 120,000.

The actual number of refugees is likely much higher, the UN agency said, because many people who fled Syria don't officially register until they are out of resources. A UN report said it's believed still others choose not to register because of "fear that names, photos, and personal information may be misused.