Police stand guard at a Christmas market in Berlin, Germany, Dec 19, 2016 after a truck ploughed into the crowded Christmas market in the German capital. [Photo/Agencies]

German Chancellor Angela Merkel said authorities are acting on the assumption that the truck ramming that claimed 12 lives and injured 48 in Berlin on Monday was terror-related.

Police initially questioned a 23-year-old Pakistan-born man who was suspected of intentionally plowing a seven-ton truck into a crowded Christmas market in the German capital. However, after questioning, he was released and the manhunt continued for the person responsible for the attack.

"It would be very difficult for us to learn that a human being who came to Germany to ask for refuge and asylum committed this deed," Merkel said in an address on national television. "It would be terrible for all Germans who are very active, day by day, in helping asylum seekers and refugees."

Among those confirmed dead was a Polish national found in the cab of the truck. Police apprehended the initial suspect driver 2 km from the scene after a witness allegedly followed him on foot.

The ramming adds to mounting tension in Europe after a spate of terror-related incidents.

On Monday, the Russian Ambassador to Turkey, Andrey Karlov, was shot and killed in Ankara by an off-duty policeman named as 22-year-old Mevlut Mert Altintas. The Russian foreign ministry has declared the shooting a terror attack. The assassin was shot dead by Turkish special forces.

Also on Monday, a man shot and wounded three people at an Islamic center in Zurich — police later found the shooter dead nearby. They have not yet released information linking that attack to terror groups.

China’s Foreign Ministry spokeswoman Hua Chunying offered condolences to the families of the Russian ambassador and other victims, and added that the ministry was trying to ascertain whether any Chinese nationals were harmed in the Berlin attack.

Li Wei, director of the Institute for Security and Arms Control Studies at the China Institute of Contemporary International Relations, expressed concern that such attacks would be on the rise next year, due to unresolved conflict in Syria and Iraq.

Thirty-two civilians and three perpetrators died in a series of coordinated suicide bombings in Brussels in March. In July, a Tunisian-born man drove a truck through a crowd in Nice, France, killing 84 and injuring 202 — the worst terror attack on French soil since the November 2015 attacks that caused 180 deaths and more than 360 injuries in the Paris region.

A UK government security official, speaking on condition of anonymity, said the attacks in Germany and Switzerland followed the latest model of terror attacks.

"Because ISIS is being forced to retreat in the Middle East, they no longer have such a strong infrastructure which allows them to initiate coordinated attacks such as those in Paris and Belgium," he said. "They’ve opted for what we call the lone wolf style of attack, much harder for intelligence services to track."

Li Xiaokun in Beijing and agencies contributed to this story.