The European Union is rushing to bolster security at energy installations after the apparent act of sabotage on two major Russian pipelines.
The head of the Commission has joined other world leaders in calling the leak an act of sabotage - but it has stopped short of directly accusing Russia.
Pictures released by the Danish military show gas bubbling to the surface of the sea, stretching one kilometre across the Baltic Sea.
Denmark says it could take up to a fortnight before the area is calm enough for close inspection.
Its foreign minister Jeppe Kofod is adamant the ruptures were caused by foul play.
Hamburg's University of the Federal Armed Forces researcher Julian Pawlak spoke to Guyon Espiner.
The EU is proposing a further round of sanctions on Russia to put to its 27 member states.
The Commission's head, Ursula von der Leyen, says if approved, the trade bans would deprive Russia of an additional 7 billion euros in revenue.
The moves come amid expectations Russia will annex four-occupied eastern provinces in Ukraine, after strong support from a so-called "referendum".
