More than 20 Ukrainian children with cancer have been airlifted to the UK, the government said on Monday, as an appeal began for Britons to take in refugees in their homes.

Health Secretary Sajid Javid said the 21 children had been receiving treatment in Ukraine but were forced to leave their homes because of Russia's invasion.

They are now being given "life-saving" care by the state-run National Health Service (NHS) and have been accompanied by their carers, he told Sky News television.

The government in London has been criticised for insisting that those fleeing the conflict and wanting to join family in the UK have to apply for visas to be able to travel.

Its insistence on security checks and visas has earned it unfavourable comparisons with the European Union, which has allowed Ukrainians visa-free stays for up to three years.

As of Saturday, "just over 3,000" visas had been granted under a UK scheme for family members, Javid told Times Radio.

On Monday the government launches its "Homes for Ukraine" programme, which aims to allow "tens of thousands" of Ukrainians without family ties to stay in the UK.

Hosts will be given 350 pounds ($457, 418 euros) a month and have to commit to housing refugees for a minimum of six months, senior minister Michael Gove said on Sunday.

But additional overheads could prove problematic: Britons are grappling with the worst cost-of-living crisis in a generation, as energy bills and inflation soar.

Javid said the sick children, who arrived in the UK from Poland on Sunday evening, were initially given six-month visas but will be allowed to stay "as long as necessary".

They were brought to the UK with the help of St Jude Children's Research Hospital, a US organisation which specialises in the most serious paediatric illnesses