Spanish authorities say they have seized hashish with a street value of around 20 million euros ($22 million) from a merchant ship that allegedly was smuggling the drug from North Africa to the eastern Mediterranean.

A Spanish government statement Tuesday says aircraft spotted several semi-rigid boats, which are often used by North African drug-runners, around the Moldovan-flagged ship ELG. Agents boarded it and found more than 10 metric tons of hashish in the hold.

The statement says the ship's six Ukrainian crew were arrested in Monday's operation about 100 miles (160 kilometers) southeast of Spain.

Authorities began tracking the ship six months ago.

Officials say large amounts of hashish travel from an area near the Strait of Gibraltar to countries in the eastern Mediterranean.