According to the Times, the army-for-hire, run by oligarch Yevgeny Prigozhin - a close ally of the Russian president who is often dubbed 'Putin's chef' - was flown in five weeks ago and is being offered a huge sum for the mission.

The highly-trained operatives are said to be waiting for the green light from the Kremlin to pounce, with their hit list also including Ukraine's prime minister, the entire cabinet, mayor of Kyiv Vitali Klitschko and his brother Wladimir - both boxing champions who have become iconic figures on the front lines of the capital.

However the plans were rumbled after they reached the upper echelons of the Ukrainian government on Saturday morning.

It prompted Kyiv to declare a 36-hour 'hard' curfew, ordering everyone indoors so that soldiers could sweep the streets for Russian saboteurs.

Citizens were warned that they risked being 'liquidated' if they were spotted outside during the curfew hours, as they may be assumed to be the enemy.

A source with knowledge of the Wagner Group's activities told the Times that between 2,000 and 4,000 mercenaries had actually arrived in Ukraine in January, but with different missions.

Some were sent to the rebel-held regions of Donetsk and Luhansk in the east of the country - while the 400 tasked with taking out Zelensky headed to Kyiv from Belarus.

The group are said to be tracking Zelesnky and his colleagues via their mobile phones - claiming to know where they are at all times.

Another source claimed the killers-for-hire have been told to put on the brakes while Putin holds 'peace talks' with Zelensky this week.

But they have allegedly been informed that the Russian leader has no desire to reach a deal, branding the meeting on the Belarus border as 'smoke and mirrors.'

News of their assassination plans has not seemed to faze Zelensky, who admitted he was 'target number one' during an address to the nation, telling them Russian special forces were hunting him.

When the US offered to extract him, he told President Joe Biden: 'I need ammunition, not a ride.'

Wagner has conducted covert operations across Africa and the Middle East, including in Syria, and they have most recently been on the ground in Ukraine to guide Russian tanks to the capital.

General Sir Richard Barrons, a former commander of Joint Forces Command, said: 'They are very effective because they are hard to pin down.

'They can appear from the shadows, do very violent things and then disappear again, without it being obvious who was responsible. They are not directly linked to the Russian government and therefore they are plausibly deniable.'

Sources told the Times the militia were briefed about Putin's plans on Ukraine back in December, long before the Russian army was told.

Soldiers and generals were feared to oppose the invasion plans, and several have been executed for doing so, claimed the mercenaries.