"We are where we are, there is no movement at this point," Xinhua news agency quoted Esper as saying in an interview on Thursday, adding "where they are going with S-400s is the wrong direction".
The remarks were made after this week's North Atlantic Treaty Organization (NATO) summit held in London, where Esper took part in meetings with the Turkish side.
During the summit, US President Donald Trump said the two sides remained in talks on the issue, noting that prior to Ankara's decision to purchase the S-400 missile system, it was denied the opportunity to buy the American Patriot missile system.
The S-400 missile system has been a major issue in the US-Turkey ties after Ankara signed a $2.5 billion agreement with Russia to purchase S-400 missile batteries in 2017, and began taking deliveries in July this year.
In response, Washington removed Ankara from its F-35 fighter jet programme, in which Turkey is a buyer and manufacturer, for fear that the move might enable Russia to obtain sensitive information on the aircraft.
Earlier this month, two US Senators issued a letter urging sanctions against Turkey in a move less than a month after the November visit by Turkish President Recep Tayyip Erdogan to the White House.
( With inputs from IANS )