A map showing an approximate route of the Grace 1 tanker, which authorities in Gibraltar say was headed for Syria. Sea-Seek.com

An Iranian oil tanker seized in the Mediterranean on Thursday had taken an unusual route, going all the way around Africa rather than using the Suez Canal.

It was seized off Gibraltar, where authorities say the tanker, called the Grace 1, was carrying Iranian oil to Syria in violation of European Union sanctions.

According to an intelligence source cited by Reuters, the route, which is about 10,000 miles longer, could have been an attempt to avoid being caught in the Suez Canal, where security is greater.

An Iranian tanker seized by British authorities on Thursday had taken a detour of thousands of miles around Africa in what could have been an attempt to avoid scrutiny and sneak thousands of tons of crude oil to Syria.

Authorities in the British territory of Gibraltar seized the Grace 1 tanker on Thursday, saying they thought it was taking its cargo to a government-owned oil refinery in Syria, an act that would violate European Union sanctions.

According to Reuters and the Associated Press, tracking data showed that the ship took an unusual route to the Mediterranean.

Leaving Iran, it traveled all the way around Africa before being stopped at the Mediterranean's western entrance. Had it traveled all the way to Syria, the journey would have been about 14,000 miles, according to the shipping-route website Sea Distances.

Read more: British Royal Marines seized a tanker full of Iranian oil over claims it was breaching sanctions by sailing crude oil to Syria

A much more direct route from Iran to anywhere in the Mediterranean would be via the Suez Canal in Egypt, which links the Red Sea and the Mediterranean.

That journey would have been just over 4,000 miles, according to Sea Distances.

The Grace 1 oil tanker seen off Gibraltar on Thursday after being boarded by the Royal Marines. AP Photo/Marcos Moreno

The distances involved are estimates, as Business Insider does not have access to precise location data for the vessel.

A likely reason for the odd route could be the lesser chance of being caught going the long way.

A maritime intelligence source told Reuters that the ship may have avoided the Suez Canal because that route involved unloading its cargo, where it would run the risk of being caught and seized.

This is what this route would have looked like:

This map shows the route through the Suez Canal, which would have been much shorter. Google Maps/Business Insider

The European Union has banned oil shipments to Syria since 2011 as part of sanctions over President Bashar Assad's treatment of civilians.

Iran says its ship has been illegally detained, and it has summoned the UK ambassador. The UK central government and the US praised Gibraltar's action.

Reuters said it had seen shipping data suggesting the ship was loaded with oil in Iran, though the ship's documents say it was loaded in Iraq.

The ship was seized in Gibraltar — far from Iran and Syria. Google Maps/Business Insider

The ship was recently tracked in Iran's Bandar Assalyeh port, according to Reuters.

An analyst at Kpler, a London-based energy-data firm, told Reuters the ship loaded Iranian crude oil at Iran's Kharg Island port in mid-April.

Spain's caretaker foreign minister said the tanker was stopped at the request of the US, which has seen tensions increase with Iran in recent weeks as it tightens sanctions around Iran's oil exports.