The United States is set to begin charging import duties of 25 percent on steel and 10 percent on aluminium on Friday, although appears to be ready at the last minute to consider exemptions beyond those already granted to Canada and Mexico.
By the time EU leaders meet for the summit at 2 p.m. (1300 GMT), European Trade Commissioner Cecilia Malmstrom will have returned from talks with U.S. Commerce Security Wilbur Ross and U.S. Trade Representative Robert Lighthizer in a bid to secure an EU exemption.
She is expected to brief EU ambassadors and the European Parliament on Thursday morning, before the summit. The signs late on Wednesday pointed towards a willingness to find a solution rather than an impending trade war.
Lighthizer told the U.S. Ways and Means Committee that the United States was in talks with the European Union, Argentina and Australia on granting possible exemptions to metals tariffs, and hoped to settle the issue by the end of April.
Malmstrom and Ross said they had agreed to start immediate discussions on areas of trade concern, including steel and aluminium.
Trump's final word is awaited, but Donald Tusk, who chairs EU summits, said he was cautiously optimistic.
"Meanwhile leaders will discuss how to respond to President Trump's overall approach to global trade, which could negatively affect jobs all over the world. If the United States turns protectionist for good, the whole world will have a problem," he told a news conference on the eve of the summit.
"We are not there yet and there is still time to act sensibly."
The European Commission has proposed that, if tariffs are imposed, the bloc should challenge them at the World Trade Organization, consider measures to prevent metal flooding into Europe and impose import duties on U.S. products to "rebalance" EU-U.S. trade.
The one word neither the Commission nor EU leaders will want to use is "retaliation".
"The EU is quite conscious of not wanting to fight the U.S. or give Trump a bloody nose through words," one diplomat said.
The EU leaders' second topic on Thursday, taxation, also threatens to expose transatlantic strains.
The European Commission on Wednesday proposed rules to make digital companies pay more tax, with U.S. tech giants such as Google, Facebook and Amazon set to foot a large chunk of a potential 5 billion euro (£4.4 billion) bill.
EU Economics Commissioner Pierre Moscovici brushed off accusations that he was going after rich American tech companies to enrich EU coffers and France, Germany, Italy, Britain and Spain welcomed the proposals in a joint statement.
However, some smaller countries fear the proposed tax would undermine their ability to attract multinationals and see the measure as more likely to shift tax revenue to bigger EU countries rather than raising more money.
(Reporting by Philip Blenkinsop, Gabriela Baczynska and Alastair Macdonald; Editing by Alison Williams)
By Philip Blenkinsop