The U.S. and the EU are stepping up pressure on Turkey over concerns that the country's banking sector is a potential loophole through which Russia can evade sanctions, the FT reported, citing sources.

The U.S. has turned its attention to Turkish banks working with the Russian payment system Mir. At the same time, Brussels is preparing a delegation to voice its concerns directly to Turkish officials.

The pressure on Turkey has come as Western countries have moved from imposing new sanctions to tightening existing restrictions. "We need to close the loopholes," another source said.

A total of five major Turkish banks - Vakıfbank, Ziraat Bank, İş Bank, DenizBank and Halkbank - are connected to Russia's Mir payment system.

According to officials, in addition to Turkey, measures against potential loopholes to evade sanctions are aimed at the countries of the Caucasus, Central Asia and the Persian Gulf.