Late on Monday, Obama said Assad would face consequences if he crossed the "red line" of even moving unconventional weapons in a threatening way.

"There would be enormous consequences if we start seeing movement on the chemical weapons front or the use of chemical weapons," Obama told reporters in Washington, D.C. "That would change my calculations significantly."

Syria revealed in July that it has chemical weapons and said it could use them if there was "external aggression" brought into the Syrian conflict.

Obama's warning comes after UN observers left the capital, Damascus, following a failed four-month mission to implement a UN Security Council peace plan drawn up by former UN special envoy Kofi Annan, who resigned earlier this month.

Japanese journalist killed

Meanwhile, Japan's Foreign Ministry says that journalist Mika Yamamoto was killed in a gunfight between Syrian forces and rebels in Aleppo on Tuesday. The award winning-reporter worked for the Tokyo-based news agency, Japan Press. She was 45 years old. An unconfirmed report from the British-based Syrian Observatory of Human Rights said an additional three journalists were missing in Syria as well.

As fighting continued in Syria on Tuesday, the violence extended over the border into Syria's western neighbor, Lebanon, as well.

According to security and army officials in the northern part of the country, gunfire between pro- and anti-Syrian regime groups that began overnight left nearly 30 people injured near the city of Tripoli on the Mediterranean coast.

mz/ipj (AFP, Reuters, dpa)