On Wednesday, Gazprom said it would halt gas deliveries to Ukraine while Kiev said it could find cheaper supplies elsewhere.
The letter, written on Thursday and signed by Bulgaria, the Czech Republic, Estonia, Greece, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia, says the project should come under the closest regulatory scrutiny and called for "an inclusive debate" at next month`s EU summit.
"The position of the European Commission on the Nord Stream II project will also essentially influence the perception of the EU`s common foreign and security policy among its core allies and traditional partners," the letter, seen by Reuters, said.
World leaders have been urging Moscow and Ankara to avoid any escalation of tensions following the shooting down of a Russian jet by NATO member Turkey.
Russia`s relationships with the European Union and Ukraine have been volatile since the annexation of Crimea in March last year.
The Nord Stream extension to deliver increased volumes of gas straight to Germany could have serious consequences for Kiev and EU nations, the letter said.
"Preserving the transport route through Ukraine is the strategic interest of the EU as a whole, not only from an energy security perspective, but also reinforcing the stability of the Eastern European region," it said.
The Commission wants to reduce the bloc`s dependence on Russia, which supplies about a third of its energy.
It has already said it will scrutinize the Nord Stream extension project for conformity with EU law and that its priority is to diversify suppliers while maintaining Ukraine`s status as a transit nation.
"Ukraine is a safe transit route. Gas should continue to flow through Ukraine," European Climate and Energy Commissioner Miguel Arias Canete told reporters on Thursday.
He added the Commission was not unduly concerned by the cutoff of Ukraine`s gas supplies - a recurrent event - as stocks were comfortable.
More about:
