Russia may already have personnel in place in Ukraine to assist with an invasion, the UK Government believes.
Defence Secretary Ben Wallace told MPs there were individuals already in Ukraine "linked to the Russian state in ways that are not conventional" and "that should give cause for concern".
He added: "We are becoming aware of a significant number of individuals that are assessed to be associated with Russian military advance force operations that currently are located in Ukraine."
It comes as concerns over a potential Russian invasion into Ukraine continue to increase.
Moscow has massed an estimated 100,000 troops near the border with Ukraine.
Mr Wallace told the Commons Foreign Affairs Committee that any incursion into Ukrainian territory would be unacceptable.
He said: "Any crossing into Ukraine, whether small or large, would be viewed as a breach of that sovereignty, against international law and an invasion.
"You can't be half-pregnant, you are either invading a country or you are not."
Watch: What happens if NATO triggers Article 5?
Earlier, the Prime Minister said the UK plans on being part of any new NATO deployments if Russia was to invade Ukraine.
Boris Johnson told MPs: "The British Army leads the NATO battlegroup in Estonia and, if Russia invades Ukraine, we would look to contribute to any new NATO deployments to protect our allies in Europe."
He was asked by former British Army colonel-turned Tory MP James Sunderland "to comment on the feasibility of direct military action by NATO notwithstanding that Article 5 does not apply" if Ukraine was invaded.
Mr Johnson replied, saying: "There is no member of NATO that is currently willing to deploy in Ukraine in large numbers to fight Russian aggression in the way that he suggests."
But the PM said the UK must "beware of doing things that would constitute a pretext for Putin to invade".
"We have to calculate and calibrate what we do very carefully and I think building a strong package of economic sanctions, continuing to supply defensive weaponry and all the other things that we're doing, that's the right package."
Ukraine is not a full NATO member but was granted Enhanced Opportunities Partner status in 2020.
Despite this, Ukraine is not included in NATO's vital Article 5 where "an armed attack against one or more [NATO ally] in Europe or North America shall be considered an attack against them all".