London, Mar 15 (IANS): Russian forces may only be able to sustain full fighting capacity for another "10 to 14" days, senior UK defence sources indicated, after which President Vladimir Putin's men will struggle to hold the ground they have already captured from Ukrainian troops, the Daily Mail reported.

UK defence sources say that Kiev has Moscow "on the run" and the Russian army could be just two weeks from "culmination point" - after which "the strength of Ukraine's resistance should become greater than Russia's attacking force".

Advances across Ukraine have already stopped as Moscow's manpower runs short, the report said.

President Volodymyr Zelensky echoed that optimism in an early-morning address to the Ukrainian people, saying his military continues to inflict 'devastating losses on Russian troops.'

"Soon the number of downed helicopters of Russia will reach hundreds of units. They have already lost 80 warplanes. Hundreds of tanks and thousands of other units of equipment. In 19 days, the Russian army has lost more in Ukraine than in two bloody and years-long wars in Chechnya," he added.

But, as Russia's invasion falters, its methods become more brutal, with cities increasingly coming under indiscriminate rocket fire.

Kiev, the capital, suffered another round of bombing on Tuesday morning as apartment blocks were set on fire by early-hours strikes, Daily Mail reported.

Ukraine's military said four Russian helicopters, a jet, and a cruise missile were shot down by its forces which remained in control of all major cities, including the badly-hit southern port of Mariupol and the entire region of Kherson.

Putin's stuttering invasion has forced even his close allies to admit, publicly, that things are not going to plan.

Russian National Guard chief Viktor Zolotov, once in charge of Putin's personal security, admitted on Tuesday that "not everything is going as fast as we would like".

But he still insisted that Russia would achieve victory "step by step".