The U.S. has applied to seize an Iranian oil tanker detained by British marines last month.
The Grace 1 is being held in Gibraltar.
It was seized on suspicion of carrying oil to Syria, in defiance of European sanctions.
The move sparked an escalating dispute between between London and Tehran.
On July 19, Iran seized a British tanker in apparent retaliation.
The Grace 1 was expected to be released Thursday (July 15).
Its captain and three officers have been set free.
There were expectations that Iran might then release the UK vessel.
But the U.S. Department of Justice stepped in.
UK officials confirm it has applied to seize the Grace 1.
It says that's based on a number of allegations, without giving any detail.
The move may scupper hopes of an easing in tensions.
EU nations have been trying to save Iran's nuclear deal with international powers.
Washington has pulled out of the agreement, but Europe wants to keep it alive.
Tensions over the tanker threatened that.
Now it seems that problem isn't about to go away.