As does NATO chief Jens Stoltenberg:
British Foreign Secretary Boris Johnson offers his condolences:
U.S. Embassy in Russia offers condolences to victims and their families of the St. Petersburg subway blast.
Kremlin pool journalist Dmitry Smirnov posts video of Russian President Vladimir Putin at his meeting with Belarusian President Alyaksandr Lukashenka commenting on today's "tragedy."
Some background on previous attacks from our news desk:
There was no immediate claim of responsibility.
Suicide bombers have struck several times in Moscow and other Russian cities in the last two decades, with insurgents based in Chechnya or other parts of Russia's North Caucasus often blamed for or claiming reponsibility for the attacks.
In the most recent major attack of that kind, two suicide bombings on successive days in December 2013 killed more than 30 people in the southern city of Vologograd.
A bomb blast killed 27 people on a train en route from Moscow to St. Petersburg in 2009, but there have been no major attacks in St. Petersburg, Russia's second-largest city.
Latest update from our news desk:
Andrei Kibitov, a spokesman for the St. Petersburg governor’s office, told state-run Rossia-24 television that "according to prelinary information, 10 people were killed" in the blast, which triggered immediate fears of terrorist attack.
Kibitov said on Twitter that around 50 people were injured and that 17 emergency response teams had been mobilized. More would be activated later, he said.
Reaction from the President of the Parliamentary Assembly of the Council of Europe (PACE) to the St. Petersburg subway blast.
Russia's General Prosecutor has declared the St. Petersburg subway blast a terrorist attack.