Former Boeing executive Patrick Shanahan took over at the Pentagon after serving as deputy, thrust onto the world stage when his predecessor Jim Mattis quit after long-running disagreements with Trump.

His first appearance for the cameras at NATO headquarters in Brussels, a routine "grip and grin" handshake with alliance Secretary General Jens Stoltenberg, drew unusual interest, with Shanahan's every word and gesture picked over for meaning.

"It's a pleasure to be over to spend time with you and especially in a such a nice facility," said Shanahan, after a tour that has taken in Kabul and Baghdad as well as Brussels in just a few days.

"One of the other things that I am looking forward to during the time that we are together is to talk about the future of NATO, the capabilities that we can further develop."

With nerves still jangling in Europe after July's tumultuous summit, when Trump reportedly threatened to "go it alone" unless allies boosted their defence spending, Shanahan's reference to "the future of NATO" sent some journalists into a spin.

"Really? They want to start dismantling NATO?" exclaimed one.

US officials said Shanahan was simply referring to upcoming NATO initiatives to fight terrorism and emerging threats to alliance security, which will be discussed at a leaders' meeting in London in December.

At his press conference later in the day, Stoltenberg said Shanahan was "very well received by all allies".

"It was a very strong and a very clear message about the US ironclad commitment to NATO... also the very strong personal commitment he has to NATO," Stoltenberg said.

- European doubts -

Mattis was highly regarded in NATO and in the run-up to his departure there was anxious talk in Europe about what might happen if he quit.

Former NATO secretary general Jaap de Hoop Scheffer described Mattis as "one of the last men standing" in a Washington increasingly dominated by Trump's "America first" ideology.

A diplomat from a European country said they would be "listening carefully" to Shanahan to see if he would follow in the footsteps of Mattis or toe the president's line.

Former Marine general Mattis quit after clashing with Trump over the president's abrupt decision to pull US forces out of Syria, and referred to NATO twice in his resignation letter.

"Our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships," he wrote.

"NATO's 29 democracies demonstrated that strength in their commitment to fighting alongside us following the 9-11 attack on America."

NATO insists Washington's commitment to the alliance is not in doubt, pointing to increased US investments of troops and resources in Europe.

But European doubts linger about Trump's commitment to alliances generally and NATO in particular -- fuelled by a recent New York Times article suggesting the president has repeatedly told aides he does not see the point of NATO and thinks the US should withdraw.

"We don't know if Shanahan will become the new defence secretary or if he will follow Mattis's approach," said the European diplomat.

Another said: "I hear he is very close to Mattis and said he will follow this basic path. On the other hand, he has to look at the president."

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