As for the chemical weapons issue, the Syrian Foreign Ministry denied that the country has any plans to use such arms, state TV reported. The government has also repeatedly stressed it will not use such weapons, if they exist, against its people under any circumstances.
But U.S. officials say "worrying signs" suggest otherwise.
Syrian forces have started combining chemicals that could be used to make deadly sarin gas for weapons to attack rebel and civilian populations, one U.S. official said.
The intelligence, obtained over the weekend, the official said, came from multiple sources. But the official declined to provide more details about how the United States learned of it. Sarin gas, the source said, could most readily be used to fill artillery shells.
Obama said Monday that "the use of chemical weapons is and would be totally unacceptable. And if you make the tragic mistake of using these weapons, there will be consequences, and you will be held accountable."
Israeli Prime Minister Benjamin Netanyahu said that Israel and the world community are "closely monitoring the developments in Syria regarding the stockpiles of chemical weapons."
"I heard the important things President Obama said on this matter, we are of a single mind. It is forbidden to use these weapons and it should be forbidden to pass them on to terror elements," he said.