The presidents of nine NATO countries in central and eastern Europe declared on Sunday they would never recognise the annexation by Russia of Ukrainian territory.
Their reaction comes two days after Russian President Vladimir Putin signed treaties to annex four Moscow-occupied regions of Ukraine -- Donetsk, Kherson, Luhansk and Zaporizhzhia -- following "referendums" the West has dismissed as "sham".
The presidents issued a joint statement saying they could not "stay silent in the face of the blatant violation of international law by the Russian Federation".
"We reiterate our support for the sovereignty and territorial integrity of Ukraine," they said.
"We do not recognise and will never recognise Russian attempts to annex any Ukrainian territory."
The statement was issued by the presidents of the Czech Republic, Estonia, Latvia, Lithuania, Montenegro, North Macedonia, Poland, Romania and Slovakia.
Four of the signatories -- Poland, and the three Baltic states -- are on NATO's eastern flank with Russia.
Two others -- Romania and Slovakia -- have borders with Ukraine.
Hungary, which also borders Ukraine, was notably absent from the list. Its nationalist prime minister, Viktor Orban, has sought close ties with Putin in recent years and railed against European Union sanctions on the Kremlin.
Also absent were Albania, Bulgaria, Croatia and Slovenia.
The statement, published on the website of the Polish president's office, said the leaders of the signatory countries had "visited Kyiv during the war and witnessed with their own eyes the effects of Russian aggression".
"We support Ukraine in its defence against Russia's invasion, demand Russia to immediately withdraw from all the occupied territories and encourage all (NATO) Allies to substantially increase their military aid to Ukraine," it said.
"All those who commit crimes of aggression must be held accountable and brought to justice."
The presidents said they stood by a decision NATO made 14 years ago, supporting Ukraine's wish to join the trans-atlantic military alliance at a future date.
They did not comment on Russia's annexation of Ukraine's Crimean peninsula in 2014, or on Ukraine's request last Friday for fast-track NATO membership following Russia's annexation manoeuvre.
NATO members have hesitated at accepting a country at war -- which, by treaty, would oblige the alliance to come to its defence.
NATO's Article 5 says an attack on one member is tantamount to attack on all.