Mark Esper also stressed that the US will “consider” Turkey rejoining to the F-35 programme if Ankara gets rid of the S-400 system and “take them completely out of the country.”
Russian S-400 air defence system is seen as one of the most advanced missile systems in the world, capable of tracking several targets simultaneously.
"There could be more sanctions to follow, but frankly, what we’d really like is the S-400 not to become operational," US Secretary of State Mike Pompeo told Bloomberg in an interview.
US President Donald Trump's comments came a day after the White House said the Turkey's purchase of Russian air defence system made Ankara's presence in the F-35 programme impossible.
'Turkey will continue to evaluate other alternatives,' says Defense Industries Presidency head Ismail Demir criticising US' announcement to remove Turkey from the F-35 programme.
NATO Secretary-General Jens Stoltenberg, speaking at the Aspen Institute's annual security forum in the United States, said he was concerned at Turkey's expulsion from the F-35 programme.
Turkey seeks to have an integrated and complex layered air and missile defence system, which will be developed and built up both by national and foreign firms.
Turkish President Recep Tayyip Erdogan says Turkey is aiming to develop the defence industry and protect peace in the region.
Former Pakistan intelligence head General Ehsanul Haq and former deputy chief of Indian Air Force Kapil Kak say US defence missiles are no match to tested Russian defence system. Washington has warned Turkey and India against buying S-400s.
"Turkey fulfills all NATO requirements ... We strongly reject linking this matter with the F-35 [fighter jet programme]," Turkey's Defence Minister Hulisi Akar says.
For the first time in decades, the United States’ supremacy in missile technology is at risk, as world powers compete over new technology that could entirely redefine the battlefield
"We discussed the S-400 subject with Russia. Indeed the S-400 issue is settled," Turkish President Erdogan was cited as saying. "I think they will start to come in the first half of July."
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