Members of the Ranger Regiment, part of the British Army's new Special Operations Brigade, flew out on warplanes that also delivered anti-tank weapons to Ukraine.
Since the beginning of the week, there have been several flights of RAF C-17 transport aircraft. However, there is no official confirmation from either side yet.
In addition, the UK has reportedly donated its Next Generation Light Anti-Tank Weapons (NLAW), which manufacturers describe as "the first missile system in history to quickly destroy any main battle tank with just one shot from above."
Britain is also considering sending hundreds more troops to Ukraine's neighboring NATO countries to act as a "deterrent" for Russia.