Earlier on Friday the EU's trade chief said the bloc expects to be excluded from USA steel and aluminium tariffs but will go to the World Trade Organization to impose its own measures if Washington presses ahead.
European trade commissioner Cecilia Malmstrom met U.S. trade envoy Robert Lighthizer in Brussels on Saturday for what she described as "frank" discussions which "brought no immediate clarity".
U.S. President Donald Trump set import tariffs on Thursday of 25 percent on steel and 10 percent on aluminium, to come into force in 15 days.
The European Union and Japan, the United States' top economic and military ally in Asia, also reiterated that their exports were not a threat to USA national security, rejecting Trump's justification for imposing the tariffs.
Canada and Mexico will be exempt from the tariffs initially, and Australia is optimistic that it too will gain exemption.
European Commission chief Jean-Claude Juncker on Friday threatened to hit big-name U.S. brands such as Harley Davidson motorbikes and Levi's jeans with import duties, prompting Trump to fire back a threat to tax cars from the EU.
Tusk's tough words came after the European Union unveiled a raft of countermeasures that will hit a huge range of flagship United States exports - from jeans to motorbikes to cranberries - with duties if Trump acts on his threat to impose heavy tariffs on steel and aluminium.
She said discussions would continue next week.More news: Spy poisoning: Military deployed in Salisbury as police investigate graveyard
Japanese Trade Minister Hiroshige Seko said he had expressed Japanese concern to Lighthizer and warned of major market disruption.
"We call for calm-headed behaviour", Mr Seko said. Mr Seko said he believed there was still time for Japan to secure an exemption.
"We hope that we can get confirmation that the EU is excluded from this", EU Trade Commissioner Cecilia Malmstroem told policy makers, experts and reporters at an event in Brussels. Japan has decried the "grave impact" the Trump measures could have on the world economy. "We will look at the impact on Japanese businesses and make a final decision".
Japan's government has warned the measure could hurt its economic relations with the U.S. But ahead of Saturday's talks, Seko also cautioned that "falling to exchanges of unilateral measures will not be in the interest of any country", according to the Kyodo news agency. However, it took on more urgency after Trump's tariff move.
Key U.S. trading partners and businesses have warned the tariffs could backfire, provoking a trade war and hurting allies like the European Union and Japan more than China, their main target.
"We're not looking for a trade war", Ross said.
Commission Vice President Jyrki Katainen warned Washington on Friday not to expect any concessions to win an exemption.