Greece continues to increase tensions with provocative acts in the Eastern Mediterranean, most recently by testing its Russian S-300 air defense missile system during a military drill on the Greek island of Crete, security sources said Monday.

Greece tested the S-300s in a military exercise on Nov. 23-27 at the Crete Shooting Range. Germany, the Netherlands and the U.S. also participated in the drill.

The drill was conducted with the Russian S-300s and various other air defense systems, including TOR-M1, OSA-AKM, Hawk, ASRAD and the Stinger man-portable air-defense system (MANPAD). Moreover, anti-aircraft missiles and Russian, American and German-made medium- and short-range missile defense systems were tested and fired during the drill.

Security sources told the Sabah daily that Greece’s actions pressurize already strained tensions in the region. Turkey and Greece are at odds over a variety of issues, including maritime borders and drilling activities in the Eastern Mediterranean.

The use of S-300s and other Russian air defense systems in a drill by a group of NATO allies is also notable, as the U.S. has voiced strong objections to Turkey's purchase of Russian S-400s, despite refusing to sell Ankara its Patriot missiles.

The sources said Turkey's “S-400s will be used just as S-300s are used within NATO” and criticized the U.S.’ hypocrite stance.

Ties between NATO allies Turkey and the U.S. were badly strained last year over Ankara’s acquisition of the advanced S-400 Russian air defense system, prompting Washington to remove Turkey from its F-35 Lightning II jet program.

The U.S. argued that the system could be used by Russia to covertly obtain classified details on the Lockheed Martin F-35 jets and is incompatible with NATO systems. Turkey, however, insists that the S-400 would not be integrated into NATO systems and would not pose a threat to the alliance.

However, the Russian-made S-300 system has been sold to 20 countries, including NATO member countries such as Bulgaria, Greece and Slovakia.

The S-300 system, completed in 1978, is designed to defend against short- and medium-range air attacks and is considered one of the world’s most powerful air defense systems.

In 1996, Greece signed a deal with Russia for the purchase of S-300s for deployment on Greek Cypriot soil.

These missiles could not be deployed in southern Cyprus as a result of Turkish pressure, but in 1998, they were deployed on Crete, whose strategic importance has been rising steadily.

Greece signed new agreements with Russia in 1999 and 2004 to purchase TOR-M1 and OSA-AKM (SA-8B) medium- and low-altitude air defense systems.

These Russian-made air defense systems are currently an integrated part of the air defense system of Greece and have also been deployed by the Greek Cypriot administration.

NATO, similarly to the U.S., argues that alternatives to the S-400s should be found. In October, Secretary-General Jens Stoltenberg reiterated that it is a national decision what kind of defense capabilities different allies acquire. “But at the same time, what matters for NATO is interoperability and the importance of integrating air and missile defense, and that cannot be the case with a Russian system S-400,” he said.

Ankara has repeatedly stressed it was the U.S.' refusal to sell it Patriot missile systems that led it to seek other sellers, adding that Russia had offered a better deal, including technology transfers. Turkey even proposed setting up a commission with the U.S. to clarify any technical issues.