British Prime Minister Boris Johnson on Monday said the UK has "significant concerns" over Russian activity in the Crimea and on the Ukrainian border.

In a phone call to Ukraine President Volodymyr Zelensky, Johnson "reaffirmed his unwavering support for Ukraine's sovereignty and territorial integrity," his office said in a statement.

He expressed his solidarity with President Zelensky's government "in the face of this destabilising activity and commended Ukraine's approach to the situation," it added, saying the UK would work to "ensure there is no further escalation."

In a tweet after the call Zelensky thanked Johnson for his support and said the Ukraine was "not alone" and was "supported by the G7 nations," referring to the elite club of advanced economies.

The call between the British and Ukrainian leaders follows messages of solidarity from the US and the European Union after Kiev last week accused Moscow of massing thousands of military personnel on its northern and eastern borders, as well as on the Crimean peninsula annexed by Moscow in 2014.

European Union foreign policy chief Josep Borrell on Sunday pledged the bloc's "unwavering" support for Kiev.

US President Joe Biden in his first call with Zelensky affirmed Washington's support for "Ukraine's sovereignty and territorial integrity in the face of Russia's ongoing aggression".

And US forces in Europe raised their alert status in response to the "recent escalations of Russian aggression in eastern Ukraine".

Ukraine on Monday denied reports that its forces had killed a five-year-old child in an attack on pro-Moscow eastern separatists, after Russia said it would launch an investigation.

Downing Street said that Britain, which assumed the presidency of the G7 earlier this year, has agreed to continue its commitment to a training mission known as Operation Orbital, established in 2015 following Russia's annexation of the Crimea.