US General Mark Milley warns it will be 'very difficult' to eject Russian President Vladimir Putin's forces from war-torn Ukraine in 2023

The illegal invasion - triggered on February 24 by Russian President Vladimir Putin under the guise of a "special military operation" - is about to enter its 12th month.

Tens of thousands of soldiers from both sides have been killed in that time, with many more innocent residents dead or displaced who were caught in the conflict.

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But United States General Mark Milley has added to concerns the aggression will not end this year, telling reporters it was not realistic Ukraine will win the war.

Gen Milley, the Chairman of the Joint Chiefs of Staff and principal advisor to President Joe Biden and the Secretary of Defense, flagged the leader of the free world and many European leaders believe the only way the conflict will end is through negotiation.

He spoke about how there is still "significant fighting" happening in large parts of the Ukraine, with frontlines stretching from the second largest city of Kharkiv down south to Kherson and north-east to Bakhmut where Russia claims to have re-gained ground.

"From a military standpoint, this is a very, very difficult fight," he said at the US Air Force base in Ramstein, Germany, on Friday (local time), where talks between 54 nations had continued to discuss a new security package for Ukraine.

"Its more or less a static frontline right this minute, with the exception of Bakhmut and Soledar where there is significant offensive action going on from both sides."

Gen Milley estimated one of the frontlines was similar to the distance between Washington D.C. and Atlanta, or about 1,000 kilometres in length.

"That is a significant amount of territory and in that territory are still remaining a lot of Russian forces in Russian occupied Ukraine," the 64-year-old military officer warned.

"So, from a military standpoint, I still maintain that from this year it would be very, very difficult to militarily eject Russian forces from every inch of Russian-occupied Ukraine."

"That doesn’t mean it can’t happen, doesn’t mean it won’t happen but it will be very, very difficult."

Ukraine President Volodymyr Zelensky addressed the meeting in western Germany and urged allies to "act fast" with military assistance, reiterating the importance of tanks.

Germany has come under pressure to supply its Leopard 2 tanks to Ukraine but the country's defence minister was yet to approve the request as it must receive permission from Berlin on any re-export of the military vehicles by other countries, such as Poland.

It's estimated about 2,000 of these tanks are sitting in warehouses across Europe.

However, Germany would prefer if the vehicles were part of a NATO package for Ukraine, and preferably included the United States' M1 Abrams tank.

Washington has held out on the request, as training must be provided to use it, with the vehicle also requiring different fuel and regular maintenance required.

Russia has returned serve and warned a delivery of the tanks to its neighbours would be seen as an "extremely dangerous" escalation.

President Zelensky took to social media after the conference where he doubled down on his plea of acquiring the tanks to defend themselves.

He thanked allies for their support but flagged "hundreds of thank you are not hundreds of tanks".

"We must speed up! Time must become our common weapon, jut like air defense, artillery, armoured vehicles & tanks. The Kremlin must lose," he wrote to Twitter alongside a six-minute video of himself speaking to other leaders at the gathering.