The UK Chancellor Jeremy Hunt has said the UK has "very little confidence" in Russia's narrative around the Moscow concert hall attack, which it has linked to Ukraine.
He spoke as Russia declared an official day of mourning to commemorate the loss of at least 137 lives in the attack, the deadliest in Russia in two decades.
"We know that they are creating a smokescreen of propaganda to defend an utterly evil invasion of Ukraine, but that doesn't mean that it's not a tragedy when innocent people lose their lives," Mr Hunt told Sky News on Sunday.
"I take what the Russian government says with an enormous pinch of salt ... after what we've seen from them over the last few years," added Mr Hunt.
Gunmen went on a rampage on Friday at the Crocus City Hall in a suburb on the northern edge of Moscow. The ISIS terrorist group's Afghanistan affiliate has claimed responsibility for the attack.
Russian authorities have not yet blamed the group, but President Vladimir Putin has said 11 suspects have been arrested, including the four gunmen. Mr Putin claimed they were detained while attempting to travel "towards Ukraine where, according to preliminary information, they had a window to cross the border".
Russia's FSB security service said earlier that the assailants had been "in contact" with people in Ukraine as they tried to flee the country.
Mr Hunt warned that the UK and other European countries should "absolutely" be concerned about the re-emergence of ISIS on the world stage.
"We are very lucky in this country that we have incredibly impressive intelligence agencies who have been successful ... in foiling a lot of terrorist threats over recent years, but we have to remain vigilant," he said.
"If it is Islamic State, they're utterly indiscriminate in what they do, they're prepared to murder in the most horrific way."
Pope Francis on Sunday joined international condemnation of the attack, saying it was a "vile" attack that offended God.
"I assure my prayers for the victims of the vile terrorist attack carried out in Moscow, may the Lord receive them in his peace, comfort their families and convert the hearts of those who ... carry out these inhuman actions that offend God," the Pope said in St Peter's Square after the Palm Sunday mass.
Tajikistan has pushed back against claims by Russian media and politicians that the gunmen held Tajik nationality.
"Terrorists have no nationality, no homeland and no religion," President Emomali Rahmon told Mr Putin in a phone call, the Tajik president's office said in a statement on Sunday.
