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A Russian  - which are waging a brutal military campaign in Syria - have been sent to Ukraine to murder the top ranks of the government, the English daily reported on Monday.

Over 400 Russian mercenaries are thought to be operating in Kyiv and thousands more in the rest of the country, according to the daily, with Ukrainian authorities  a 36-hour 'hard' curfew in Kyiv after reports of the death squad emerged.

Their mission is to assassinate the Ukrainian President, , and his government, in addition to civilian figures leading the resistance to the Russian invasion. 

The mayor of Kyiv told civilians to stay indoors from Saturday afternoon until Monday at dawn, or risk being seen as 'saboteurs' and eliminated.

The mercenaries are part of the feared , a private military group with close ties to Russian President Vladimir Putin, which has been accused of numerous war crimes in Moscow's fight supporting brutal Syrian dictator Bashar Al-Assad.

The mercenaries were allegedly flown into Ukraine five weeks ago in preparation for the Russian invasion.

The Wagner Group has through its involvement in Syria, Libya, the Central African Republic, Mali and separatist regions of eastern Ukraine. It is widely considered an arm of Russia's foreign policy and its mercenaries have been

Russia launched a full-scale and unprovoked invasion of Ukraine on 24 February, deploying tens of thousands of troops in and around the country.

It included bloody assaults on several major Ukrainian city, including Kyiv, with rockets and artillery. 

Putin also ordered Russia's nuclear forces into high alert on Sunday, which drew international outrage. An estimated 368,000 Ukrainians have already fled to neighbouring countries according to the United Nations.