Russian President Vladimir Putin privately said he could invade Poland, Romania and the Baltic states, according to a record of a conversation with his Ukrainian counterpart.
"If I wanted, in two days I could have Russian troops not only in Kiev, but also in Riga, Vilnius, Tallinn, Warsaw and Bucharest," Mr Putin allegedly told President Petro Poroshenko of Ukraine, reported 'Suddeustche Zeitung', a German newspaper.
If true, this would be the first time Mr Putin has referred to invading NATO or European Union members. Any serious threat to send Russian troops into the capitals of Latvia, Lithuania, Estonia, Poland and Romania would cause grave alarm among Western leaders.
All five countries mentioned in this alleged conversation are covered by the security guarantee in Article V of NATO's founding treaty, which states that "an attack on one is an attack on all". In Tallinn earlier this month, US President Barack Obama confirmed NATO's commitment to this doctrine.
Mr Putin's alleged threat bears similarities to remarks he made to Jose Manuel Barroso, the president of the European Commission, in which he warned: "If I want to, I can take Kiev in two weeks".
The EU recently announced more sanctions against Russia, focusing on the energy, financial and arms sectors. NATO has agreed to form a rapid-reaction force in response to fears of a Russian threat, and stepped up air patrols over the Baltic.
Meanwhile in Washington yesterday, it was announced that the US will provide €40m in new security assistance to the Ukraine's military, but will stop short of fulfilling an urgent request from Mr Poroshenko for weapons to help his country fight against Russian-backed separatists.
Mr Poroshenko pleaded his case during remarks at a rare joint meeting of Congress. While he thanked the US for the non-lethal equipment it is providing to his country's beleaguered military, he said more was needed to stop the provocations near the Russian border.
Mr Putin made his alleged remarks in series of telephone conversations with Mr Poroshenko over the current ceasefire in eastern Ukraine.
Mr Putin also warned Mr Poroshenko not to put too much faith in the EU, saying that Russia could exert its influence and bring about a "blocking minority" among member states.
Earlier this week, Ukraine ratified a historic Association Agreement with the EU, placing the country on the path towards eventual EU membership. It was the refusal of the former president, Viktor Yanukovych, to sign this agreement last year that triggered the Ukraine crisis.
The EU recently announced further sanctions against Russia, focusing on the energy, financial and arms sectors. But there have been divisions among member states over sanctions, with many worried about the impact on their own economies.
The Baltic states are particularly nervous about Russian intentions, and Mr Obama sought to reassure them with his speech. (© Daily Telegraph London)