What's behind the seizure of an Iranian oil tanker?

Iran urges Britain to release vessel detained off Gibraltar

An Iranian oil tanker is at the centre of a growing international dispute.

British marines boarded and detained the ship on Thursday as it was sailing near Gibraltar, a British territory on Spain's south coast.

The UK believes it was violating European Union sanctions by carrying Iranian oil to Syria.

Spain says it was the United States that ordered the vessel be stopped.

Iran condemned what it called an illegal interception.

So what's behind this dramatic move? And as tension increases between the U.S. and Iran, is Europe caught between a rock and a hard place?