Until now, the U.S. President did not resort to a military involvement in the Syrian conflict, but Obama said: "there would have enormous consequences if we see the chemical or biological weapons moved to the front or being used against the rebels." He added: "That would change my calculations considerably."
"The United States monitors the situation very closely," Obama said.
"The issue of chemical weapons is aimed not only Syria. It affects our close allies in the region, including Israel. We will not have the situation where chemical or biological weapons are falling into the hands of the wrong people," Obama said.
Calling again for Assad to step aside to allow a democratic transition, Obama conceded that prospects for a "soft landing" to the conflict were dim.
Last month, Obama warned the regime that it would make a tragic mistake and should be held accountable if it used weapons of mass destruction.
"We want Assad and his entourage to know: The world looks at you, and you will be held accountable before the international community and the United States, if you make the tragic mistake of using chemical weapons," Obama stressed.
According to the U.S.-based Global Security website, there are four suspected chemical weapons sites in Syria producing the nerve agents VX, sarin and tabun.
Meanwhile, the U.S. State Department spokesperson Victoria Nuland said that Washington doubts that the Security Council will manage to reach a consensus on a Syrian settlement.
"Syrian President Bashar al-Assad should relinquish power," she said.
French President Francois Hollande said Monday after a meeting with new international envoy to Syria, Lakhdar Brahimi, that there is no political solution for the unrest in Syria unless Assad leaves power. "There can be no political solution without the departure of Bashar Assad," he stressed.
He also reiterated France's commitment to a free and democratic Syria, a country that would be respectful of the rights of each of its sects.
On the ground, fighting raged on, with 167 people killed across Syria on Monday, the Syrian Observatory for Human Rights said.
Clashes in the district of Jobar, in the east of Damascus, following the free Syrian Army fighters attack against regime forces checkpoint .
The Syrian regime announced over a month ago they had regained control of Damascus, but fighting has taken place over the past week, especially in the southern and western areas of Damascus.
In Aleppo, a Japanese journalist, Mika Yamamoto, died of wounds sustained there, a Syrian activist group said in a statement.
It seems that the U.S. president is well aware of the situation on land, in Syria. He is also very sure about Assad's will to use chemical and biological weapons, therefore Obama's threats should be taken into consideration, especially after getting information about the possibilty of using weapons of mass destruction by the Syrian regime in order to end the ongoing popular revolution.
War still continues in Aleppo and Damascus. The announcement of the regime saying they control most of Syrian land is untrue.