BERLIN, April 5 (Reuters)

German Chancellor Angela Merkel will visit U.S. President Donald Trump on April 27, a senior U.S. official said on Thursday, as differences over a nuclear deal with Iran and trade cast a shadow over the transatlantic relationship.

Merkel's trip, three days after French president Emmanuel Macron's state visit to the U.S. capital, will come just before the expiry of an exemption for the European Union from U.S. import duties on steel and aluminum.

The twin visits would give the European Union's two leading national leaders the opportunity to lobby for the bloc to be exempted permanently from the steel and aluminum tariffs. The tariffs are suspended for the EU until May 1.

Merkel's visit to the United States, first reported by mass-selling daily Bild, will also take place shortly before a May 12 deadline that Trump has set to improve an international deal to curb Iran's nuclear program.

Her office had no immediate comment on the planned visit.

Merkel, in a telephone call with Trump last week, urged dialogue on trade policy between the EU and the United States, "taking into account the rules-based international trade system".

Merkel's relationship with Trump got off to a frosty start after his November 2016 election.

Before a phone conversation on March 1 to discuss the war in Syria and Russian nuclear arms, the two leaders had not spoken to each other for more than five months.

Trump has threatened to withdraw the United States from an accord between Tehran and six world powers, signed in 2015 before he took office, unless France, Britain and Germany help to agree a follow-up pact by that date. Trump does not like the deal's limited duration, among other things.