Russian leaders are trying to "identify and punish" those responsible for Friday's terrorist attack at a concert hall on the outskirts of Moscow. They have blamed Ukraine and its Western allies for being involved, but they are also toughening their policy towards migrants.

Islamic State militants claimed responsibility for the attack, which killed 143 people and wounded more than 300. Authorities charged four men who are believed to be from Tajikistan.

Local media say security services have been conducting raids over the past few days on migrant communities, particularly those from Central Asia. They quote human rights lawyers as saying they have received more than 1,000 calls for help from people who have been detained or tortured.

Tajikistan's embassy has warned its citizens in Russia to avoid public gatherings. Kyrgyzstan's foreign ministry is telling its people to "temporarily refrain" from traveling there.

Still, Russian leaders say it is "extremely difficult to believe" Islamic militants have the capabilities to organize such an attack by themselves.

Foreign ministry spokesperson Maria Zakharova said at a news conference on Wednesday that Western leaders are placing all the blame on the Islamic State group, but they are "behind these banned terrorist organizations."

Russian authorities say Ukraine, the US and the UK were behind the attack. British Foreign Minister David Cameron said on social media that those claims are "utter nonsense."