European Commission head Jean-Claude Juncker warned Moscow Wednesday the security of EU member nations was sacrosanct, amid growing alarm in eastern states over the crisis in Ukraine.
"The security and borders of EU member states are untouchable. I want this to be understood very clearly in Moscow," Juncker told the European Parliament in his first State of the Union address.
"Poland and the Baltic states and others ... are very important members of the EU and they should not think that (in a crisis) we would not be there," he said.
"We have a duty to protect the security of everyone."
Russian support for pro-Moscow rebels in eastern Ukraine and its annexation of Crimea have badly spooked newer eastern European EU members such as Poland, Latvia, Lithuania and Estonia who fear Russia wants to reassert its Cold War-era control over them.
Juncker said these countries "have to know that solidarity is not an empty word. We are there now and we will be there if ever there is a need."
According to the text of his speech, which was not read in full, Juncker said that while the EU should "keep our nerve" on tough Ukraine sanctions against Russia, the bloc should not close the door to Moscow in efforts to solve a conflict which has cost nearly 8,000 lives.
"The EU must show Russia the cost of confrontation but it must also make clear it is prepared to engage," he said in the text.
Broad EU economic sanctions against Russia are due to expire at the end of January.
The EU has no armed forces of its own to command and defence is solely a national responsibility but Juncker has been a long-term backer of the idea of a European army.
He also told MEPs that the bloc had to have a more assertive foreign policy to make sure its voice was heard in solving the many conflicts such as in Syria and Libya which directly affect its security and well-being.