European countries are at loggerheads with Washington over punishing new US tariffs on steel and aluminium as well as President Donald Trump's decision to pull out of the Iran nuclear deal and the Paris climate accord.

NATO defence ministers will meet in Brussels on Thursday to approve plans to beef up the alliance's ability to mobilise forces quickly in the event of a crisis, as concern about the threat from Russia shows no sign of abating.

They will also sign off on two new command centres -- one to protect Atlantic shipping lanes, based in Norfolk, Virginia, and another to coordinate troop movements around Europe, located in the southern German city of Ulm.

But the meeting, just five weeks out from a full summit of NATO leaders, looks set to be overshadowed by the spat between European countries and their longtime ally across the Atlantic.

"There are now serious disagreements with NATO allies on serious issues," Stoltenberg told reporters, saying he was working hard to minimise the fallout for the alliance.

"As long as they're not solved I have to be focused on how to reduce, limit the negative consequences for NATO."

- 'Strong together' -

Stoltenberg, who held talks with Trump in Washington last month, insisted the "transatlantic bond" remained strong and pointed to the way NATO survived major differences among members over the Iraq war in 2003 and the Suez crisis of 1956.

"What we have seen again and again is that we have been able to unite around NATO's core task, to protect and defend each other despite those differences," he said.

US Defense Secretary Jim Mattis told reporters that it was too early to start talking about a trade war -- "you know, there is always give and take on these things."

"What we are looking for is fair and reciprocal trade," Mattis said en route to Brussels, adding: "I don't anticipate any effect on the security arena."

He said he was going to Brussels "to listen to them, to take notes, to work these things forward and to find a common ground to work at."

The leaders of all 29 NATO members will meet in Brussels for the summit in July when all eyes will be on Trump, who has repeatedly attacked European countries for not pulling their weight in the alliance.

Germany, Europe's largest economy, has suffered the worst of Trump's wrath for its failure to live up to a pledge by all NATO countries to try to spend at least two percent of gross domestic product on defence.

US neighbour Canada is also upset at being included on the tariff list and has rejected a US proposal to go further and slap steep tariffs on car imports over American national security concerns.

"I will be very vocal" about the trade fight, said Canadian Defense Minister Harjit Sajjan. "To consider Canada as a national security risk as a result of steel is beyond ridiculous."

- 'Four 30s' plan -

NATO is modernising its command structure and beefing up its defences in response to growing fears about Russia, following Moscow's annexation of Crimea in 2014.

Thousands of NATO troops are already stationed on standby in the Baltic states and Poland as deterrent and on Thursday defence ministers will give the go-ahead to a new US initiative called "30-30-30-30" or "four 30s".

Under the plan, by 2020 NATO will have 30 battalions, 30 air squadrons and 30 warships ready to be used within 30 days to back up existing rapid response forces.

"This is about establishing a culture of readiness and we need that because we have a more unpredictable security environment, we have to be prepared for the unforeseen," Stoltenberg said.