European Commission chief Jean-Claude Juncker said US efforts to split the EU on trade "are in vain," in a message Wednesday to President Donald Trump before their talks in Washington next week.
Juncker, who heads the powerful executive arm of the 28-nation European Union, will visit the White House on July 25 to try to convince Trump to drop tariffs on metals and avoid further duties on cars.
He told a press conference that he "would like to make the US president understand" a key point on trade while admitting he had tried unsuccessfully with him before.
"The European Union and its single market are indivisible," the former Luxembourg premier said when asked about Trump's bid for bilateral deals.
"All the efforts to divide the Europeans are in vain," Juncker said.
"It seems to me all these efforts to have bilateral ties, which must take place between our partners and the commission and therefore the EU, must be better explained," he added.
"I go there calmly," he said.
The Commission said Juncker and Trump will focus on improving transatlantic trade and forging a stronger economic partnership.
But Washington has shown no sign of compromise as it pursues its "America First" policies across a range of issues, even if it means alienating traditional allies.
Trump imposed duties of 25 percent on steel imports and 10 percent on aluminium in March, but had given temporary exemptions to the EU and some other close trading partners as talks continued.
But those exemptions were lifted in early June, prompting a raft of retaliatory tariffs by the EU, and Trump is now considering whether to impose hefty levies on European automobiles.
Besides the tariffs, the US has threatened European companies with huge fines if they continue to operate in Iran, after Trump reimposed far-reaching sanctions while pulling out of the landmark 2015 deal on curbing Tehran's nuclear programme.
A French finance ministry source told AFP on Tuesday that US officials have rejected a joint request made by Paris, London, Berlin and the EU last month to exempt their companies operating in Iran from the sanctions.
