President Xi Jinping sent a letter of condolence to Russian President Vladimir Putin on Tuesday over the St Petersburg metro explosion.

Xi said he was shocked to learn that the metro explosion resulted in major casualties, and that China strongly condemns the terrorist attack.

"On behalf of the Chinese government and Chinese people, and in my own name, I would like to offer profound condolences to the deceased and sincere sympathies to the injured and families of the victims," he said.

He said the Chinese people would stand firmly with the Russian people at this difficult moment.

China firmly opposes terrorism in all forms, and is willing to step up cooperation on counterterrorism with Russia and the international community to safeguard peace and security in the region and the world, he said.

At least 11 people were killed and 45 others were wounded in a blast in the St. Petersburg subway on Monday, the Russian news agency sputniknews reported.