US President Barack Obama warned Syria of "enormous consequences" if it resorts to chemical weapons, as regime forces launched a wave of air and ground attacks and a Japanese reporter was killed in Aleppo.

Regime fighter jets and shells struck towns in northern Syria and areas in Damascus on Tuesday, killing nine people including women and children, the Syrian Observatory for Human Rights said.

Japanese female reporter Mika Yamamoto, 45, died after coming under fire from up to 15 apparently pro-government troops as she covered the anti-regime movement in the key northern hub of Aleppo on Monday, a colleague said.

Her death brings to four the number of foreign journalists who have lost their lives in Syria since the uprising against the regime of President Bashar al-Assad began in March 2011.

Obama put Assad's regime on notice Monday that although he had not ordered military intervention "at this point," the United States was "monitoring the situation very carefully," and had drawn up contingency plans.

"There would be enormous consequences if we start seeing movement on the chemical weapons front or the use of chemical weapons... That would change my calculations significantly," he told reporters at the White House.

Obama -- who has repeatedly called for Assad to stand down as the conflict intensified -- said the United States would regard any recourse by Damascus to its deadly arsenal as crossing a "red line".

Syria's admission in July that it has chemical weapons and could use them in case of any "external aggression" added a dangerous new dimension to a conflict that began as a peaceful uprising but has descended into a bitter armed revolt.

On Tuesday, Syrian forces shelled districts in the battered city of Aleppo, killing nine civilians, among them women and children, and pounded the nearby towns of Marea and Tall Rifaat, the Syrian Observatory for Human Rights said.

The Local Coordination Committees -- a network of opposition activists on the ground -- said warplanes were also striking Marea, causing casualties when one building collapsed.

"Residents are trying to pull out the martyrs and the wounded from the rubble," the LCC said.

Aleppo, the main northern city which lies near the Turkish border, has borne the brunt of the conflict since fighting erupted there a month ago, with the regime warning it would be the scene of the "mother of all battles".

Tuesday's violence followed a bloody day in which 167 people were killed across Syria, the Observatory said, with no letup in the bloodshed as Muslims celebrated the Eid al-Fitr holiday which marks the end of the holy month of Ramadan.

The LCC also reported heavy artillery shelling from tanks at a checkpoint in Jdaidet Artuz, southwest of the capital and said warplanes were firing from mounted machineguns on the suburbs of al-Hajar al-Aswad and Babila.

Syria's popular uprising has spiralled into an all-out armed conflict between rebel fighters and regime forces with more than 23,000 people killed, according to the Observatory. The United Nations puts the death toll at 17,000.

Yamamoto's long-time collaborator Kazutaka Sato told Japanese broadcasters he and the veteran war reporter had been with anti-regime forces when they were shot at by what appeared to be a group of about 15 government troops. The TBS network cited Sato as saying she had been shot in the neck.

Abu Raashid, commander of Liwa Asifat al Shamal (Northern Storm Brigade), one of the myriad that make up the rebel Free Syrian Army, said his group took Yamamoto's body across the border into Turkey.

"If the international community doesn't move to help the Syrian people, they have to react to the spilling of their citizens' blood on Syrian territory," he told AFP.

Yamamoto worked for the small Japan Press news agency and had also covered the war in Afghanistan and the Iraq conflict, according to the company website.

The Observatory said three other journalists -- a Lebanese woman, an Arab male working for a US media outlet and a Turkish national -- were missing.

But Syria -- which insists it is fighting an insurgency by "armed terrorist groups" backed by the West, Gulf states andFrench reporter Gilles Jacquier was killed on January 11 in the central city of Homs, where American journalist Marie Colvin and French photographer Remi Ochlik both perished on February 22.

Lakhdar Brahimi, who has replaced former UN chief Kofi Annan as the international point man for Syria, warned on Sunday that it was now a matter of ending civil war rather than avoiding it.

Turkey -- reacted angrily saying that to speak of civil war "contradicts reality".

Assad appeared in public at Eid prayers on Sunday, but his regime is increasingly embattled after a wave of high-level defections and the killing of four top security chiefs in a July bomb attack.

The conflict has also raised fears of a spillover into neighbouring countries, which are sheltering several hundred thousand refugees who have fled the fighting, and Turkey has warned it cannot take many more.

The United Nations has warned of a humanitarian crisis with more than one million people also displaced inside Syria and up to 2.5 million in need of aid.