Vladimir Putin has committed “nearly 100 per cent” of the forces his military had previously amassed on the border of Ukraine into Russia’s unprovoked invasion, a US defence official said.

       The senior official, speaking on background, briefed reporters on Monday morning as Russia’s invasion of Ukraine continues to stretch on with no end in sight and reports indicating that Russian advances have stalled across much of the country.

       That makes the latest developments all the more significant as the official added that US intelligence had no indication yet that Russia’s military had plans to reinforce the invasion with further troop deployments.

       “We have not seen any indication that [Putin] is going to go beyond the amassed combat power that he’s already developed,” the official said, according to a Politico transcript of the call.

       Defence Department spokesman John Kirby confirmed at his briefing later in the day that the frustration of Russia’s advances by Ukraine’s military was leading to morale issues across Russia’s front line.

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       "It is not clear to us that all of the soldiers that Russia has put into Ukraine realised that that's what they were doing,” he told reporters.

       "[T]hey are having morale problems, they are having supply problems, they are having food problems, they are having fuel problems,” Mr Kirby continued.

       Spokesman John Kirby speaks during a briefing at the Pentagon

       (AP)

       While Russia is not reportedly seeking to augment its invasion force with additional units of the Russian military at this time, Mr Kirby did add on Monday that the Pentagon saw “some truth” to reports that Moscow was seeking to hire Russian-aligned fighters from Syria for the war effort.

       "We do believe that the accounts of the Russians seeking Syrian fighters to augment their forces in Ukraine, we believe there's truth to that,” said Mr Kirby, calling the decision an “interesting” move by Mr Putin.

       The US announced the deployment of a few hundred additional troops to bases around Europe on Monday as Nato allies seek to shore up their defences against Russia should Moscow’s invasion spill over into Ukraine’s neighboring countries.

       Mr Kirby reiterated on Monday that the US and Nato will not use its forces to defend Ukraine’s territory even as the West has pressed forward with efforts to better arm Ukraine’s military.