Image copyright Getty Images Image caption Russia was the second-biggest exporter of aluminium to the US after Canada last year

The US has filed a complaint against Russia at the World Trade Organisation (WTO), challenging the retaliatory duties that Moscow has placed on US imports as trade tensions rise.

It is the latest US move in the tit-for-tat trade war that began in March with US tariffs on aluminium and steel.

The US says the protections are allowed under national security grounds.

It argues that the retaliatory measures violate trade rules because they are too high and apply only to US imports.

The US has filed similar complaints against the EU, Canada, China, Mexico and Turkey, which are among the countries that have retaliated against the US metals tariffs.

Those countries are among those that have also brought their own complaints against the US over the import duties.

The US started to collect a 25% border tax on foreign steel and a 10% border tax on foreign aluminium in March, with some exceptions. The metals had accounted for about $48bn in imports.

The EU, Canada and Mexico were initially exempted from the measures amid negotiation, but that reprieve ended in June.

They retaliated by raising taxes on certain US goods. All told, the retaliation so far applies to about $24bn in US goods.

Russia's retaliation, which went into effect this month, affects almost $90m in US products, including items such as construction equipment.

National security?

It is not clear how the WTO will rule on the various cases.

In theory, its rules do allow for certain national security exceptions, but the justification has been rarely invoked until now.

In addition to the metal tariffs, the Trump administration has also used national security to begin an investigation of car imports, which could lead to tariffs.

Analysts have warned that if the US succeeds in its national security case, it could spur other countries to use national security to justify protections.

Even within the US, however, politicians and other experts argue that tariffs on metals from close allies, such as Canada, bear little relation to national security.