The bombing of a subway train in Russia has drawn condemnation from world leaders.
Tributes have been left outside the station where 10 people died in the blast.
President Putin says all causes are being considered, including terrorism.
Foreign Secretary Boris Johnson said he was "horrified" by the attack.
French Foreign Minister Jean-Marc Ayrault says nation would "stand by all those who suffer."
President Donald Trump called the deadly bomb blast on a Russian subway train "absolutely a terrible thing."
Trump spoke Monday before a working lunch with Egypt's President Abdel-Fattah el-Sissi. Trump said "it's happening all over the world."
Iran's Foreign Ministry Spokesman Bahram Ghasemi said killing innocent people "is the most humiliating act for achieving political goals."
Lebanon's militant Hezbollah group, which is backing Syrian President Bashar Assad's forces along with Russia, says the incident was the type of "terrorism" Russia was fighting in Syria.
UN Secretary-General Antonio Guterres said those responsible for the bombing must be held accountable.
EU foreign policy chief Federica Mogherini sent a condolence message to Russian Foreign Minister Sergei Lavrov, saying she and her colleagues were following the news from St. Petersburg "with a lot of apprehension".