US Secretary of State Rex Tillerson attempted to strike an upbeat tone Tuesday as he embarked on his first foreign trip since the White House called his future into question.

Tillerson arrived in Brussels for talks with EU and NATO leaders trailed by reports that President Donald Trump is frustrated with his performance and plans to sack him.

Before he began his official program in Brussels, Tillerson met US diplomats serving in the city to reassure them about his much-criticised efforts to renew US policy.

He admitted that work to recruit new senior talent to the State Department to deliver Trump's agenda had been slow, but he insisted his reform agenda was picking up speed.

And, while he has not yet completed any of his main policy priorities, from developing a working relationship with Russia to facing down North Korea, he said progress has been made.

"While we don't have any wins on the board yet, I can tell you we're in a much better position to advance America's interests around the world than we were 10 months ago," he told staff and their families at the US embassy in Brussels.

He lavished praise on the "acting" senior officials who have stepped up to run US diplomacy as Trump's administration struggles to find staff for key positions.