PARIS, Nov 24 (Reuters) - The European Union must make it clear to Russia that there would be a high price to pay if it acted against Ukraine, Estonian Prime Minister Kaja Kallas told Reuters on Wednesday, urging the bloc to quickly agree on how to deter Moscow. The increase of military activity on both sides follows weeks of rising tension that have raised the risk of war between the two ex-Soviet neighbours, even though Moscow denies aggressive intent and Western intelligence sources have told Reuters they do not see any invasion as imminent. "We share the concern about the Russian military build-up around the Ukrainian border," Kallas said in an interview after talks in Paris with French President Emmanuel Macron. "It is therefore very important that we are clear on the European side that the price of taking any steps towards Ukraine will be so high that it will act as a deterrent and make Russia reconsider." She said there was a narrow window for the EU and the United States to agree on a common deterrent because the migrant crisis on the Belarus-Polish border, Germany"s current lack of a government and France"s approach to a presidential election could be the "perfect storm" for Russian President Vladimir Putin to test the EU"s resolve. Kallas said there were still plenty of tools in the box regarding sanctions that could make Putin think twice. Estonia shares a border with Russia and last week summoned 1,700 reserve soldiers for an unannounced military exercise which includes installing a razor wire barrier along 40 km of its border with Russia. Kallas said that having seen fellow Baltic states Latvia and Lithuania react with toughened border security facing Belarus after migrants were pushed across the Belarusian frontier into Poland, Tallinn did not want to be caught out. "If this is a kind of weapon that someone has in mind to use against us, then we are making the preparations." There were no signs at this stage of any threat to Estonia"s border or of migrants being shuffled towards Estonia through Russia, Kallas said. Estonia has no border with Belarus.