The concern about Russian arms sales to Azerbaijan appeared to have influenced Armenian President Serge Sarkisian more than any other issue, Charles Tannock, Member of the European Parliament, told the BBC.
"Since the 1990s Nagorno-Karabakh war Armenia has had close military co-operation with Russia, and without that Armenia would be totally vulnerable to attack by Azerbaijan", Mr Tannock said.
Armenian President Serge Sarkisian held talks with Russian President Vladimir Putin in Moscow on Tuesday, and drew a direct link between trade ties and the security partnership between former Soviet states. Sarkisian spoke of Armenia's "desire to join the customs union and get involved in the process of creating the Eurasian Economic Union". Mr Putin sees a future Eurasian union as an economic bloc pulling together former Soviet states. Currently Russia's customs union only embraces Belarus and Kazakhstan.