UN Secretary-General Antonio Guterres condemned the bombing in St Petersburg subway, which killed at least 10 people, saying that those responsible for the attack should be held accountable.
"The Secretary-General condemns the bombing in the St Petersburg metro," Xinhua news agency qoted UN spokesman Stephane Dujarric as saying on Monday.
"He extends his deepest sympathy and condolences to the families of the victims and to the government and the people of the Russian Federation," Dujarric said. "Those responsible for this appalling act must be held accountable."
Meanwhile, US President Donald Trump also denounced the attack.
"Terrible. Terrible thing. Happening all over the world. Absolutely a terrible thing," Efe news quoted Trump as saying.
Expressing full solidarity with the Russian people and government in such hard times, Egypt offered condolences to the families of the victims and wished speedy recovery for the wounded.
"We express our most sincere condolences to the Russian society over the bomb blast in subway in St. Petersburg, in which innocent people were killed," a Polish ministry statement said.
President Vladimir Putin, who was in the city on Monday to meet the Belarusian President, said that all possible causes would be considered.
The blast took place on a subway train in the Russian city of St Petersburg on Monday afternoon, killing at least 10 people and injuring about 50 others, a report said.
The bomb, a homemade device filled with shrapnel, exploded in the third car just after the front of the train had entered the tunnel, the report said.