PARIS, Jan. 9-- The two main suspects of the armed attack at the headquarters of the Paris-based Charlie Hebdo satirical weekly were still at large so far, after an intensive search operation finished in the Picardie region northeast of Paris.

France has deployed tens of thousands of troops in the hunt for the two brothers, Cherif Kouachi, 32, and his 34-year-old brother Said, accused of killing 10 journalists and two policemen on Wednesday morning at the offices of Charlie Hebdo.

The manhunt came as the head of Britain's domestic spy agency MI5 warned that Islamist militants were planning other "mass casualty attacks against the West", and that intelligence services may be powerless to stop them.

In the rural Aisne region of Picardie, elite armed police and paramilitary forces backed by helicopters searched a wooded area near which the fugitives had earlier robbed a petrol station and abandoned their getaway car following Wednesday's shooting in the capital.

According to the French television BFMTV, one of its journalists witnessed the departure of police vehicles from the area searched on Thursday night. However, police presence is maintained there to deal with any eventuality.

The brothers were thought to have carried out the deadliest attack since 1995, in revenge for the weekly's repeated publication of cartoons mocking the Prophet Mohammed.

Charlie Hebdo, which is based in Paris' 11th arrondissement, was firebombed in 2011, due to the publishing of a controversial series of cartoons depicting Prophet Mohammed.

In its last published cartoons, the weekly mocked Abu Bakr al- Baghdadi, leader of the Islamic State, which seized major towns in Iraq and Syria.

One of the three suspects, 18-year-old Hamyd Mourad, turned himself in to police late on Wednesday.

According to reports, Cherif Kouachi had previously been tried on terrorism charges and served 18 months in prison.

France increased on Thursday its security alert system, Vigipirat, to attack alert in its northern region of Picardie.

Meanwhile, police patrols and security have been reinforced at public places including major stores, religious centers and public transportation systems.

On Thursday night, French Interior Minister Bernard Cazeneuve said during a press conference that nine people were put in custody as part of the investigation into Charlie Hebdo attack.

Meanwhile, the international community has strongly condemned the terror attack by all means, including holding demonstrations and protests.

Chinese President Xi Jinping on Thursday sent a message of condolences to his French counterpart, Francois Hollande, and strongly condemned the terror attack.

Terrorism is a common enemy of all mankind and a common threat to the entire international community, including both China and France, Xi said.

China is firmly opposed to all forms of terrorism, and stands ready to work with France and other countries to boost security and counter-terrorism cooperation so as to safeguard world peace and protect the people of all countries in the world, he added.

Canadian Prime Minister Stephen Harper said on Thursday that the "international jihadists" had declared war on countries around the world, but people would not be intimidated by "jihadist terrorists."

"We're just going to have to face that head on and deal with it. And that's what our government is committed to do," Harper said, warning that Canadians are not immune from terrorist threats.

U.S. President Barack Obama on Thursday visited the French embassy in Washington D.C. to pay personal condolences over the killing of 12 people, saying "As allies across the centuries, we stand united with our French brothers to ensure that justice is done and our way of life is defended."

Leaders of Singapore, Cambodia, Malta, Russia and many other countries as well as international and regional organizations, such as the African Union (AU) Commission, have all extended condolences to France, and voiced strongly condemnation over the killing.

Meanwhile, Britain has taken "precautionary" steps to increase security at the French-British border, and maintained its international terrorism threat level at "severe," the second highest of all five possible threat levels, meaning that a terrorist attack is "highly likely" and the public should remain vigilant.