U.S. President Barack Obama on Monday warned the Syrian government against using chemical weapons in its intensifying conflict with the rebels.

"Given the regime's stockpiles of chemical weapons, we will continue to make it clear to al-Assad and those around him that the world is watching," the president told a gathering of U.S. veterans at the western state of Nevada, referring to Syrian President Bashar al-Assad.

As the conflict in the Middle East country has entered its 17th month and fighting has raged for days in the capital city of Damascus and Aleppo, Syria's second largest city, there are increasing concern in Washington over the chemical weapons stockpiled in the country. U.S. officials have urged the Syrian government to secure the stockpiles.

"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," Obama warned.

Syria's stockpile of chemical weapons, which Western powers believe is one of the world's largest, is believed to include sarin nerve agent, mustard gas and cyanide.

The chemical or germ weapons would never be used in the ongoing conflict with the opposition forces, except in the case of " external aggression", the Syrian Foreign Ministry said on Monday.

Washington has been mounting "a secret but limited" effort to speed the fall of al-Assad without using force by blocking arms and oil shipments from Iran, a staunch ally of Syria, and by passing intelligence to front-line allies, the Wall Street Journal reported on Monday.