A German newspaper has quoted a private conversation between Russian President Vladimir Putin and Ukranian President Petro Poroshenko, where Putin allegedly threatened to invade Poland, Romania and the Baltic states.

German newspaper, Süddeustche Zeitung, quoted Putin allegedly telling the Ukrainian President Poroshenko:

"If I wanted, in two days I could have Russian troops not only in Kiev, but also in Riga, Vilnius, Tallinn, Warsaw and Bucharest."

All of the five countries mentioned in the alleged threat are members of both the European Union and Nato.

Article 5 of Nato's founding, North Atlantic Treaty, "commits each member state to consider an armed attack against one member state to be an armed attack against them all."

The article implies that any targeted attack on the five countries mentioned in Putin's threat could trigger a war with a number of other Western powers as well.

So far, the article has been invoked only once in NATO's history, when the United States took action following the September 11 attacks in 2001.

Putin's private threat to President Poroshenko earlier made a similar remark to the President of the European Commission, Jose Manuel Barroso, saying: "If I want to, I can take Kiev in two weeks".

The private discussion and the underlying threat were leaked after the German newspaper reviewed a European Union memorandum of the meeting between Barroso and President Poroshenko in Kiev last week.

The threats were made over telephone conversations between Putin and Poroshenko during the discussions on the current ceasefire in eastern Ukraine.

Obama reaffirms Nato's security guarantee

President Obama has reaffirmed Nato's commitment to the security guarantee, that protects all member states, when he made a speech earlier this month in Tallinn.

"If you ever ask again 'Who will come to help?' you'll know the answer: the Nato alliance, including the armed forces of the United States of America. We'll be here for Estonia. We will be here for Latvia. We will be here for Lithuania," said President Obama.

European Commission has refused to confirm the alleged threat over lack of witnesses.

"We will not conduct diplomacy in the press or discuss extracts of confidential conversations. What matters to the EU and the Commission is to contribute to lasting peace, stability and prosperity in Ukraine," said Pia Ahrenkilde Hansen, spokeswoman of the European Commission.

Critics suggest that President Poroshenko's claims might be exaggerated to garner EU and Nato support for Ukraine.