Police in southern Turkey's Mersin confiscated more than 1.9 million Captagon pills, destined to be shipped to Middle Eastern countries.

Six suspects were also detained on drug smuggling charges. They were accused of attempting to ship drugs to unidentified countries by air and sea.

The suspects smuggled drugs to Turkey from Europe, police sources said. The drugs, hidden inside large pipes, were discovered in a secret tunnel at a warehouse in Mersin. Police are now investigating the local and foreign collaborators of the suspects.

This is the third major operation in two months where police in southern Turkey seized millions of Captagon pills.

Last week, 16 suspects were arrested for smuggling Captagon pills into the Gulf countries. Police confiscated more than 1.5 million pills worth about TL 22 million ($3.76 million).

Saudi Arabia is one of the main markets for the drug. According to a Reuters report, experts estimate that some 40 percent of drug users between the age of 12 and 22 in Saudi Arabia are addicted to Captagon, a pill commonly seized in operations in Turkey.

In March, Istanbul police seized 320,000 Captagon pills bound for Saudi Arabia. Captagon, a synthetic drug made from amphetamine fenethylline, is cheap to produce and its composition includes widely available legal substances.