The controversial Nord Stream 2 gas pipeline may not be able to start operations for several months, the head of Germany's Federal Network Agency has warned.

Jochen Homann told the daily Frankfurter Allgemeine Zeitung that "further steps are missing" before a German-based subsidiary set up by Russian owner Gazprom could be added to the commercial register.

"A conclusion will unlikely be possible in the first half of the year," Homann said in an interview to be published on Monday.

The mammoth project was completed in September last year, but no gas is flowing. With tensions escalating in Ukraine, both Berlin and Washington have warned that the project could be sanctioned if Russia launches an invasion of its neighbor.

Why is there a delay?

The EU gas directive requires the operation of the pipeline and the distribution of gas to be managed separately.

Nord Stream 2 AG, based in Zug, Switzerland, had applied to the Federal Network Agency for certification last year as an independent operator. But that application did not go through, as the rules require the network operator to be registered in Germany.

The new German-based subsidiary set up by Gazprom, Gas for Europe GmbH, which is headquartered in the city of Schwerin, must now submit the documents necessary for certification. A Gas for Europe spokesman said on Wednesday that this would occur "as soon as possible."

Why is the pipeline controversial?

Nord Stream 2 is intended to deliver Russian gas to Germany via a pipeline in the Baltic Sea. It would effectively double German gas imports from Russia.

The US and a number of Eastern European countries have said it will give Russia too much leverage over European energy markets. Gas through the pipeline would also bypass Ukraine, depriving that country of much-needed transit revenue.

Berlin has long insisted that the pipeline is purely an economic issue. But the ongoing administrative delay has prompted hopes that Germany is willing to change course.

Recently, the pipeline has also been a focus of deliberations over potential sanctions on Russia should it invade Ukraine. Fears of such an invasion have been prompted by a massive military buildup at Russia's border to its neighbor.