The UK will join U.S. efforts to protect tankers passing through the Strait of Hormuz from an increasingly aggressive Iran.
British Foreign Secretary Dominic Raab says the UK has to respond now to uphold freedom of navigation.
Many countries are still cautious, though U.S. Secretary of State Mike Pompeo has once again pressed Washington's closest Asian allies to help out.
Speaking in Australia, Pompeo mentioned South Korea and Japan by name for the second straight day.
He said the U.S. wants to make sure Iran doesn't do anything risky, and wants to protect the South Korean and Japanese economies, which depend on oil and goods delivered through the region.