Russia has struggled to sustain effective offensive combat power since the start of its invasion of Ukraine and the problem is likely to become increasingly acute, British military intelligence said Tuesday.

"As well as dealing with severe under-manning, Russian planners face a dilemma between deploying reserves to the Donbass or defending against Ukrainian counterattacks in the southwestern Kherson sector," the Ministry of Defense said in an intelligence update.

The ministry added that while Russia may still make further territorial gains, its operational tempo and rate of advance are likely to be very slow.

After failing to capture the capital Kyiv early in the invasion, Russian forces used a campaign of devastating bombing and now control large swaths of Ukraine's south and east, where pro-Russian separatists already control territory.

Russia ordered military units to intensify operations to prevent Ukrainian strikes on areas held by Russia, according to Ukraine, which on the weekend reported shelling along the frontline in what it said was preparation for a fresh assault.

Russian President Vladimir Putin launched the invasion on Feb. 24, calling it a "special military operation" to demilitarize its neighbor and rid it of dangerous nationalists.

Kyiv and the West say it was an imperialist land grab and attempt to reconquer a country that broke free of Moscow's rule with the break-up of the Soviet Union in 1991.