A top military commander from the Syrian Republican Guard and close friend of President Bashar al-Assad has confirmed that he has defected from the regime.

Brigadier General Manaf Tlass, the son of former defense minister Mustafa Tlass, said in a video broadcast on Saudi-owned Al-Arabiya TV that Syrians must work together to build a new, democratic country.

He also said members of the Syrian Army should denounce "regime crimes" committed during the government's bloody crackdown on dissent.

Tlass is the highest-level defector from Assad's regime in the 17-month uprising that activists say has left more than 19,000 dead.

The video is his first public appearance since he left Syria earlier in July.

His long silence raised questions about whether he had joined the anti-Assad uprising or merely fled Syria's civil war.

General Tlass did not say where he is. Earlier unconfirmed reports suggested that Tlass fled into Turkey.

Meanwhile, after a series of setbacks for Assad's forces, government troops reportedly were trying to solidify their grip on parts of Aleppo and Damascus -- knowing that the fall of either city could spell the regime's end.

In Aleppo, Syria's largest city, reports say the government was using fighter jets and helicopter gunships to attack rebel combatants.

Farther south, Syrian Army troops were continuing to comb through districts of Damascus after the near complete rout of the largest rebel assault yet on the capital.

Activists say that, on the evening of July 24, the opposition Syrian Free Army attacked the tail end of a Syrian Army column, which was comprised of armored vehicles and thousands of government troops.

The activists say the column was composed of forces that vacated a strategic plateau in the northwestern province of Idlid and was heading toward Aleppo overnight.

'Territorial Gains'

Earlier on July 24, U.S. Secretary of State Hillary Clinton said opposition fighters were making some territorial gains that eventually could become "safe havens" and provide a base for further rebel operations against government forces.

Clinton also told reporters in Washington that it is not too late for Assad to begin planning for a political transition to end the violence.

Also on July 24, Assad appointed well-known regime figures to head his security and intelligence after a bomb attack last week killed senior officials charged with fighting the rebellion against the regime.

Analysts said those appointments suggest it is unlikely the government would end its efforts to defeat antiregime rebels.

On July 24, Syrian rebels also accused the government of moving chemical weapons to airports near the country's borders. The claim by the opposition Free Syrian Army could not immediately be confirmed.

A government spokesman said Damascus would not use chemical weapons against Syrian citizens but could use them against foreign troops if there is an international military intervention in Syria.

Reacting to the statement, U.S. President Barack Obama warned that Assad and his regime would be "held accountable" by the international community and the United States if they used chemical weapons.

Russia criticized the government threat, noting that Damascus was bound by international treaties not to use chemical weapons.

With reporting by Reuters, AP, AFP, dpa, and the BBC