Russian paramilitary units providing security for the soccer World Cup included among their members Cossack fighters who took part in clandestine campaigns in Ukraine and Syria that Kyiv and Washington condemn as backed by Moscow.

More than 800 members of at least six local Cossack organizations will patrol the streets, fan zones and team bases during the five-week tournament starting on Thursday, according to Cossack leaders and regional officials, as reported by Reuters.

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In some places in Russia, they will also work on match days as stewards or volunteers, the Cossack commanders said.

Speaking in his office decorated with flags of pro-Russian separatists fighting in eastern Ukraine, Cossack leader Alexander Anishchenko told Reuters about 200 members of the Great Don Army Cossack organization will guard areas outside the Rostov stadium during the World Cup matches.

Sochi, the Black Sea resort which hosted the 2014 Winter Olympics, will see the biggest deployment of Cossacks with 538 members of the local Kuban Cossack Army taking part, the group's spokeswoman told Reuters.

Five members of the Kuban Cossack Army have been killed in combat in eastern Ukraine and Syria since 2014, according to local Cossack commanders and people who knew the dead men.

At least 19 members of these groups have been identified by Reuters as having fought either alongside Moscow-backed separatist rebels in eastern Ukraine or as Kremlin-backed private military contractors in Syria in support of Moscow's ally, President Bashar al-Assad.

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Oleksandr Turchynov, Secretary of Ukraine's Security Council, expressed outrage over their involvement in football's premier tournament.

"The use of Cossack paramilitary groups is, on the one hand, evidence of the inability of the Russian leadership to ensure law and order during the 2018 soccer World Cup using state agencies, and on the other hand, it compromises an official FIFA event," Turchynov said in a statement to Reuters.

"I believe that it is unacceptable on the part of the international community to consent to these actions."

The statutes of FIFA, global football's governing body, state that it "is committed to respecting all internationally recognized human rights and shall strive to promote the protection of these rights."

"For FIFA, these groups' involvement represents an outrageous betrayal of the organization's charter," U.S. Congressman Eliot L. Engel, the top Democrat on the House Foreign Affairs Committee, told the publication.

"I fear that rather than providing security at a peaceful, multinational gathering, these forces will instead silence government critics, beat protesters, and crack down on anyone who doesn't conform to (President Vladimir) Putin's standards," he said in a statement.

Asked about the Cossacks' role, a FIFA spokesperson said the organization has "complete trust in the security arrangements and comprehensive security concept" developed by the Russian authorities and local organizers of the tournament.