The Kremlin said Saturday Russian President Vladimir Putin was still upset about last week's deadly attack on a concert venue on the outskirts of Moscow.

More than a week on, the Russian president has not met with the victims of the attack, claimed by ISIS-K, which left at least 144 people dead.

What the Kremlin said

Spokesman Dmitry Peskov said Putin was dealing with the tragedy, even if this was not outwardly visible.

"Believe me, if you don't see tears on his face, it doesn't mean he's not in pain. And what he is going through, it is unlikely that anyone will ever recognise and understand," he said.

He also has not visited the Crocus City Hall concert complex where the attack took place.

Diplomats lay flowers at venue

In sharp contrast, foreign diplomats, including the United States ambassador and some from the EU, laid flowers at a makeshift memorial for victims outside the venue on Saturday.

Russian state news agency RIA Novosti noted that the attendees included representatives of "unfriendly states."

Piles of flowers and toys have been laid outside the concert venue as it became a spot for members of the public to remember the victims of the attack, including hundreds of injured people.

Surge in Tajik migrant workers leaving Russia

Tajikistan's Ministry of Labour, Migration and Employment said there was a surge of migrant workers leaving Russia for Tajikistan after the attack.

"There are a lot of calls. These are most likely not so much complaints about harassment, but about our citizens' fear, panic, many want to leave. We are now monitoring the situation; more people are coming (to Tajikistan) than leaving," Deputy Labour Minister Shakhnoza Nodiri told Russia's state news agency TASS.

Four of the suspected gunmen are Tajik citizens, and several suspected accomplices also come from the Central Asian nation.

The Moscow attack has been claimed by the ISIS-K terror group, an offshoot of the self-styled "Islamic State."

Gunmen stormed into the Crocus City Hall on March 22, opening fire with automatic weapons at concertgoers and setting the building on fire in the worst attack in Russia in two decades.

Putin has acknowledged that "radical Islamists" carried out the massacre, but he has continued to insist that Ukraine was involved somehow.

A claim that Kyiv vehemently denies.

lo/rc (AP, AFP, Reuters)