NATO chief Jens Stoltenberg said Wednesday he was battling to stop the increasingly bitter row between Europe and the US spilling over into the alliance, warning of "serious disagreements" among member countries.
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 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."
Stoltenberg 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.
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.