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By Elliw Gwawr
BBC Wales Westminster correspondent
The UK government has expressed "deep concern" over a recent visit to the Ukraine by two senior Welsh politicians.
Plaid Cymru leader Adam Price and Welsh Labour government minister Mick Antoniw arrived in the capital Kyiv on Saturday, against official advice.
The trip sparked letters to Plaid Cymru and Labour from the Foreign, Commonwealth and Development Office.
Plaid Cymru and the Welsh government have been asked to comment.
The pair returned to the UK on Wednesday.
Foreign Office Minister Amanda Milling wrote to Plaid Cymru and Welsh Labour to "underline the seriousness of the situation" advising them to "leave Ukraine immediately".
In the letters, seen by the BBC, Ms Milling writes "The Foreign Office has been advising against all travel to the Ukraine since the 11th of February and recommending that all British nationals in the country should leave now.
"This advice applies equally to Members of Parliament and the Senedd".
Speaking during a meeting of the Welsh Affairs committee on Wednesday, the Welsh Secretary Simon Hart said it was an "absolutely stinging letter".
Writing on 21 February, Ms Milling asks that her concerns are passed to Mr Price and Mr Antoniw, who were in the country for four days as part of a UK delegation.
"I want to underline the seriousness of the situation: we have temporarily relocated our Embassy staff out of Kyiv and stress that any Russian military action in Ukraine would severely affect the British Government's ability to provide consular assistance to any British people there.
"The situation on the ground is unpredictable and is changing rapidly".
'Showing solidarity'
Plaid Cymru's leader has defended visiting Ukraine "to show solidarity" as fears of a Russian invasion grow.
Adam Price said he was there in a personal capacity and had paid for the trip out of his own pocket.
Welsh government Counsel General Mick Antoniw, who comes from a Ukrainian family, said: "In too many of the discussions about the situation in Ukraine, it is the people themselves who are being bypassed.
"We want to listen to what the Ukrainian people say and to show our solidarity with them.
"We stand by them and their right to determine their own future."