The United States said Monday that it was halting shipment of F-35 parts of Turkey as well as joint manufacturing work due to Ankara's purchase of the S-400 missile defense system from Russian Federation.
A day after Washington put a freeze on its joint F-35 fighter jet program with Turkey in protest to the S-400 deal, Shanahan said he expects Ankara to opt for the Patriot missiles.
"I think both the executive branch of our government, the legislative branch of our government are going to have a hard time reconciling the presence of the S-400 and the most advanced fighter aircraft that we have, the F-35", Chiefs of Staff Gen. Joseph Dunford said last month at an event at the Atlantic Council in Washington.
Mr Pence said that the country could also be expelled from the F-35 Joint Strike Fighter aircraft programme, parts of which are made in Turkey, after the USA halted deliveries of components this week.
But Turkish Foreign Minister Mevlut Cavusoglu ruled out a change of heart, saying: "The S-400 deal is a done deal and we will not step back from this".
Turkey took delivery of two F-35s at Fort Worth, Texas last June, and Turkish pilots have been training on the F-35 at Luke Air Force Base, Arizona. Two other nations that also participated in the aircraft's development - Canada, Denmark and Turkey - are scheduled to receive the F-35.
In a tweet, Turkey's vice-president later wrote: "The United States must choose".
News learned from an unnamed U.S. Department of State spokesperson that the ramifications of Ankara's purchase might end up impacting the "potential future arms transfers" to Turkey from the United States.
"I would contend that we all understand that Turkey is an important ally in the region, but it's absolutely unsustainable to support co-location of an F-35 and S-400".
Sen. Roger Wicker, R-Miss., said the planned S-400 buy was concerning and "not the action that one would expect from an ally", while Sen.
America's top commander in Europe, General Curtis Scaparrotti, was more cautious, saying he thought there was still "some possibility we can work this thing out".
The standoff escalated on Monday as the Pentagon announced it suspended delivery of equipment related to the F-35 "pending an unequivocal Turkish decision to forgo delivery of the S-400".
At the hearing, both Wolters and Townsend said their main concerns are a resurgent Russian Federation seeking to counter US influence and undermine allies and China's growing strategic and economic ties in Europe and Africa.
"We disagree with Russian Federation on many issues", Cavusoglu said, pointing to Moscow's "aggression" in the Black Sea and pledging never to recognize Russia's annexation of the Crimean peninsula from Ukraine. But US defense officials fear that Russian operators could train the S-400 radar on the aircraft, allowing them to better track it in combat.
The Trump administration has halted its sale of F-35 fighter jets to ally Turkey until the country agrees to cancel its purchase of an anti-aircraft missile system from Russian Federation, according to reports.