Prime Minister Boris Johnson has ruled out British forces fighting in Ukraine.
Speaking at a press conference at the Tapa military base in Estonia, he said: "We will not fight Russian forces in Ukraine,
"Our reinforcements like these reinforcements here in Tapa are firmly within the borders of NATO members."
He added that British forces joining the effort "would trigger a wider war with Russia".
Mr Johnson also clarified that the UK is not actively supporting British nationals volunteering to help the defence of Ukraine.
The Prime Minister did, however, offer reassurances: "We have deployed both humanitarian experts and hundreds of military logistics experts to Ukraine's neighbours to help them shelter those seeking sanctuary on their shores."
Mr Johnson also committed to doing more to allow Ukrainians to come to the UK, with around 200,000 eligible under an expanded route for people to bring family members in.
Satellite images showed the extent of the Russian forces massing around Kyiv, Ukraine's capital, with a column of armoured vehicles, tanks, artillery and support vehicles stretching 40 miles.