Russian forces in Ukraine are regrouping to double down on their attacks in the east of the country, according to Nato's secretary general.

"Russia is trying to regroup, resupply and reinforce," Jens Stoltenberg said.

The UK said Russia was bringing in between 1,200 and 2,000 of its Russian troops from Georgia as reinforcements.

Moscow said on Tuesday that it would reduce military activity in northern Ukraine and focus on "liberating" the Donbas region in the south-east.

Mr Stoltenberg said Russia's aim of pursuing a military outcome had not changed.

"We see continued shelling of cities and we see that Russia is re-positioning some of the troops, moving some of them around, most likely to reinforce their efforts in the Donbas region," he said.

"At the same time, Russia maintains pressure on Kyiv and other cities. So we can expect additional offensive actions, bringing even more suffering."

Echoing Mr Stoltenberg's remarks, the UK's Defence Secretary Ben Wallace warned that Russia's change in tactics was "not a retreat".

The UK also said bringing in Russian troops from breakaway areas of Georgia had not been part of Moscow's original plan.

"It is highly unlikely that Russia planned to generate reinforcements in this manner and it is indicative of the unexpected losses it has sustained during the invasion," the defence ministry tweeted.

Around 20% of Russian troops around Kyiv have started to reposition, according to a US senior defence official.

The official added that the Russians had largely abandoned Hostomel airport, outside of Kyiv, and are also moving away from the Chernobyl nuclear plant.

But they said Russia was keeping the ressure on in the air, with air strikes continuing to fall on the cities of Kyiv and Chernihiv in the north.

Bombs are also falling on towns and cities in the south, which is becoming the key battlefront, according to Ukrainian interior ministry adviser Vadym Denysenko, cited by Reuters news agency.

On Thursday evening Ukraine's air force appealed for more support from Western countries, saying it was operating outdated equipment that was no match for Russia's more advanced systems, had sustained losses and urgently needed modern fighter jets and air defence systems.

The air force urged provision of F-15 or F-16 fighter jets, which it said had the same advanced technology as the latest Russian fighter jets. Previous discussions had centred around Poland providing Russian-made Mig-29s but the plan was eventually dropped.

It also called for the US Patriot or Norwegian Nasams missile systems.

"Truth: Air superiority is the deciding factor in this war. Air superiority has played a key role in all wars since WWII," Ukraine's air force tweeted.

Though US intelligence reports say Russian troops have not made much progress in the east since the invasion began, Russia has said it will now concentrate its efforts on "liberating" the region.

The region is home to the self-declared and Russian-backed people's republics of Luhansk and Donetsk, which are part of Ukraine but which Russia recognised as independent three days before invading.

But Ukrainian troops have "stymied" and "frustrated" Russia's progress in the region, said the US defence intelligence official.

They added it remains to be seen whether the refitting and repositioning of troops will make a difference. But that this could mean the conflict is lengthy and more drawn out.