Trump took aim at the project which is set to run from Russia to Germany under the Baltic Sea, where preparatory work for its construction began off the German and Finnish coasts in May.

Owned by a company based in Switzerland and controlled by Russia's Gazprom monopoly, it will follow the same route as Nord Stream I, launched in 2011, which brings gas from Russia to Germany.

It is due to be completed late next year.

AFP explains the geopolitics, the political divisions and the mechanics of the pipeline in five questions:

- What's the logic behind it? -

To justify the project, the Nord Stream II company predicts the European Union will need to an extra 120 billion cubic metres of gas over the next 20 years.

With Norway and North Africa unable to boost supplies, analysts believe Gazprom will have to step in, along with liquefied natural gas (LNG) suppliers.

EU gas imports come mainly from Russia, which accounts for 41 percent of them, then Norway at 35 percent, North Africa at 12 percent and LNG suppliers at 12 percent.

Qatar is the main LNG supplier to the EU, while the share of US LNG has fallen from six percent in the first quarter a year ago to one percent this year.

- Why is Europe divided? -

The pipeline has caused splits within the EU, with Poland and other eastern states fearing it could be used as a tool to boost Russian influence over the bloc.

Nord Stream II "is an example of European countries supplying funds to Russia, which can be used against the security of Poland," Polish Foreign Minister Jacek Czaputowicz said on Wednesday.

The pipeline has been approved by Russia and three other countries whose territorial waters are being used to lay the pipeline: Sweden, Finland and Germany.

But the approval process is pending in Denmark, the fourth EU country where the pipeline will be routed through.

- What about Ukraine? -

The Baltic route allows Germany and other European countries to avoid gas piped through a vulnerable Ukraine, where pro-Russian separatists are fighting the pro-EU government in Kiev.

Russia has shut off gas supplies to Ukraine in the past, having knock-on effects in the European Union.

Transit of Russian gas through Ukraine has declined since the Nord Stream I pipeline opened in 2011.

In the first quarter of 2018, it became the main route for Russian gas to Europe, accounting for 36 percent of the total compared to 34 percent via Ukraine, EU officials said.

- Does it affect US business interests? -

Yes. Trump has backed a push by US gas suppliers to find new markets and an increase in cheap Russian gas supplies to Europe would reduce their export opportunities.

Until now US exporters have targeted markets in south America and Asia, including China, where prices are firmer.

- What's the link to NATO? -

In slamming Germany for its deal to double imports of Russian gas through Nord Stream II, Trump charged Berlin was spending billions of euros for its energy needs from Russia that it could be spending on NATO's defence.

Unlike past US administrations, Trump links Washington's commitment to collective transatlantic defence to his demand that the 28 allies pay more.

Expanding his line of attack Trump took his complaints beyond NATO member country defence budgets to an economic project that has already divided Europeans.

Analysts see the tycoon-turned-president as embracing a transactional approach to international politics rather than a strategic one against a newly assertive Russia.

The previous Barack Obama administration nonetheless pointed out the disconnect from a Germany that could sign on to EU-US sanctions against Russia over Ukraine and pursue energy deals with Moscow.