The US and Ankara, two of the transatlantic alliance's most important members, are at loggerheads over Turkey's military offensive in northern Syria, which US Secretary of State Rex Tillerson on Tuesday warned was detracting from the fight against the Islamic State group.

The two-day meeting at NATO headquarters in Brussels is expected to approve changes to NATO's command structure aimed at making the alliance fit for the challenges of warfare in the 21st century, particularly cyber tactics and hybrid warfare, as fears grow about Russian assertiveness.

But a working dinner with EU diplomatic chief Federica Mogherini on Wednesday night has taken on greater significance after senior US officials voiced fears about the bloc's defence pact and NATO Secretary General Jens Stoltenberg warned it must not undermine the alliance.

Stoltenberg said Tuesday that efforts to boost EU defence spending were welcome, but only if they were coordinated with NATO plans, warning there was "no way" the EU could replace the transatlantic alliance in guaranteeing European security.

"It will be absolutely without any meaning if NATO and the EU start to compete," the former Norwegian premier told reporters.

"European allies are absolutely aware that the defence, the protection of Europe is dependent on NATO."

The EU's so-called permanent structured cooperation on defence agreement, known as PESCO, has projects in view already to develop new military equipment and improve cooperation and decision-making.

But on Sunday a senior official working with US Defense Secretary Jim Mattis said Washington had concerns some of the proposed initiatives risked "pulling resources or capabilities away from NATO".

- Turkey row -

And on Tuesday US ambassador to NATO Kay Bailey Hutchison ramped up the pressure, warning the EU there could be serious consequences if it shuts US defence companies out of cooperation projects.

"Certainly we do not want this to be a protectionist vehicle for the EU and we're going to watch carefully, because if that becomes the case then it could splinter the strong security alliance that we have," she told reporters.

The US concerns have surprised some European diplomats, with one insisting that EU defence cooperation poses no threat to NATO, adding that "a little explanatory work" is required to clarify matters with the Americans.

Wednesday's dinner now represents an important chance for Mogherini to reassure the US, which is NATO's biggest contributor.

Potentially more serious is the festering row between the US and Turkey over Ankara's "Operation Olive Branch" launched last month against the Syrian Kurdish People's Protection Units (YPG).

While Turkey views the YPG as a "terrorist" group, the US has been working closely with the militia against Islamic State in Syria and giving it weapons, infuriating Ankara.

US ambassador Hutchison said Turkey remained an important ally and the two sides were trying to resolve the dispute.

Mattis is to meet his Turkish counterpart on the sidelines of the meeting on Wednesday.

A diplomatic source said that while the row was "a topic of concern for NATO", it was not a matter for the alliance to resolve.

"The issue will be solved bilaterally between the US and Turkey," the source said.