US Secretary of State Rex Tillerson met his European counterparts in Brussels Tuesday to shore up ties, with allies insisting he still plays a "key role" despite doubts over his future.

Tillerson held talks over lunch with European Union foreign ministers and the bloc's diplomatic chief Federica Mogherini, before a two-day NATO meeting set to focus on North Korea's missile programme and concerns over perceived hostility from Russia.

But his visit comes against a difficult backdrop -- a rift with President Donald Trump has led to reports he could be replaced within weeks, calling into question his authority to speak for Washington.

And there are major differences between Washington and Europe on a number of key policy areas, notably the Iran nuclear deal which Trump has vehemently condemned but which Brussels is desperate to preserve.

President Donald Trump's possible decision to recognise the contested holy city of Jerusalem as the Israeli capital could also prove to be thorny, after the EU warned the move could provoke "serious repercussions".

Tillerson was welcomed by Mogherini at the European Council building in Brussels, after an earlier meeting with US embassy staff.

NATO chief Jens Stoltenberg said Monday it was vital international powers worked together to tackle the North Korean crisis, after Pyongyang tested a long-range missile it said could hit anywhere in the continental United States.

"Last week's launch of an intercontinental ballistic missile showed that all allied nations could be within range," Stoltenberg said on Monday.

"The whole world needs to apply maximum pressure on North Korea in order to achieve a peacefully negotiated solution."

He said the 29-member alliance had been "clear and consistent" in its condemnation of Pyongyang's weapons programme, which has seen the reclusive state carry out a series of ballistic missile and nuclear tests in defiance of international sanctions.

The EU has been ramping up economic sanctions on the North in a bid to force it to the negotiating table -- but with no success so far.

- 'Key role' -

But if the US and EU can present a unified front on North Korea, the deal with Iran to end the Islamic republic's nuclear programme in return for the lifting of sanctions is more problematic.

Trump has slammed the historic 2015 accord, agreed after years of painstaking talks between Iran and the United States, Britain, France, China, Germany and Russia, as a bad deal and threatened to pull America out.

European powers are keen to maintain the deal and Mogherini last month travelled to Washington to lobby US lawmakers not to withdraw from the agreement.

"Preserving the nuclear deal with Iran and its full implementation is a key security priority for Europe," Mogherini said on Friday.

Tillerson's Brussels visit comes at the start of a European tour taking in Paris and Vienna for the 57-member Organization for Security and Cooperation in Europe.

Anonymous White House leaks have suggested Tillerson could be out of a job within weeks and even while denying this on Friday, President Donald Trump reminded him: "I call the final shots."

On Monday Stoltenberg gave his backing to Tillerson's efforts in tackling the North Korean crisis -- an issue where Trump has publicly criticised his top diplomat, saying he was "wasting his time" pursuing contacts with Pyongyang.

"Secretary Tillerson has played a key role, both in sending the message of deterrence, the unity and the resolve of the whole alliance, but also when it comes to the need for continuing to work for a peaceful solution," Stoltenberg said.

Tillerson has dismissed as "laughable" reports that Trump's closest aides want him to resign, but rumours were expected to dog him when he sat down with his European peers.

source: AFP