Merkel's working trip is expected to be far more business-like than this week's pomp-filled state visit by French President Emmanuel Macron, who has built a warm rapport with Trump despite stark policy differences.

Trump's lavish welcome for Macron with hugs, much back-slapping and a state dinner, contrasted sharply with a tense White House event in March last year in which Trump appeared to ignore Merkel's offer of a handshake.

Despite the more sober style of Merkel, who left for Washington Thursday, her objective will be the same as Macron's -- to persuade Trump to back off his dual threats of punitive measures that could spark a transatlantic trade war and scrapping the Iran nuclear deal.

Trump last month announced tariffs of 25 percent on steel and 10 percent on aluminium, claiming foreign imports were harming US national security by undermining the domestic production needed for military readiness.

After an outcry from US allies, Trump granted a temporary exemption to key partners including the European Union -- but this expires on May 1.

On the eve of Merkel's US visit, Berlin's hopes were dim that the EU may be spared the sweeping customs duties, to which it has vowed to retaliate, potentially setting off spiralling counter-measures.

"From today's perspective, we must assume that the tariffs will come on May 1," said a German government source. "And then we will see how we will handle it."

Trump's National Economic Council Director Larry Kudlow told CNBC Thursday that Washington would continue to exempt allied economies, including the EU, if they make concessions, citing the example of "equal treatment of automobiles".

- 'Juggling act' -

The next looming deadline is May 12, the date by which Trump has threatened to tear up the landmark 2015 agreement to stop Iran from developing nuclear weapons in return for relief from international sanctions.

Trump has called the agreement, which Germany helped negotiate, "insane" and the "worst deal ever", demanding that its "disastrous flaws" be fixed.

The EU and other signatory world powers have sought to convince Trump not to abandon the pact, warning that it was the best defence against a regional nuclear arms race.

Macron pitched the idea of seeking a separate pact to curb Iran's ballistic missile programme and support for militias in the Middle East.

The German foreign ministry has insisted that "the biggest priority is maintaining the existing nuclear accord", while Merkel has also called Iran's ballistic missile programme a "cause for concern".

US Defense Secretary Jim Mattis on Thursday said no decision had been made on whether the United States would withdraw from the Iran nuclear deal.

- 'Difficult partner' -

The talks won't be helped by the rocky relations between Germany's veteran leader and the billionaire and former reality TV star in the White House.

Trump has repeatedly berated the EU's top economy for its huge trade surplus with the United States and spending too little on NATO joint defence.

He has criticised Merkel for opening German borders since 2015 to a mass influx of mostly Muslim refugees, while she watched with dismay Trump's decision to withdraw from the Paris climate accord.

Merkel routinely praises the rules-based international order to tackle global problems from conflict and terrorism to environmental destruction, in stark contrast to Trump's "America First" stance.

"Angela Merkel is on her third US president in her long tenure as chancellor of Germany," said think-tank the German Marshall Fund of the United States.

"It is no secret that she sees the current resident of the White House as a 'difficult partner'. Her return to Washington this week, by all accounts, will be a juggling act."

Merkel -- who developed close relationships with Trump's predecessors Barack Obama and George W. Bush -- this week stressed that, despite the differences, she wants to preserve the bedrock strategic partnership.

"The transatlantic alliance, given the many non-democratic developments in this world," she said, "is a great treasure that I certainly want to cherish and nurture".

The Berliner Zeitung daily, looking ahead to Merkel's tricky diplomatic mission just after Macron's charm offensive, said that "she can hardly follow Macron's best-buddy act, but she does need a strategy".