Gasly & Hartley not guaranteed 2018 seats
"Both are Red Bull drivers, both are high skilled drivers and we want to test them for the rest of the season," said team boss Franz Tost.
"There is a high possibility that this will be the driver lineup for 2018," he revealed.
"I've not been told yet," Frenchman Gasly said in Mexico.
"I've heard I'll be in the team until the end of the season, but nothing has been said to me since Japan," he added.
And New Zealander Hartley, who is being given a second chance by Red Bull after rebuilding his career at Le Mans, also said he still needs to prove he deserves to stay.
"It's clear that the pressure is higher for me now than in Austin," he said in Austin.
"The expectations were high from the beginning but everyone understood that Austin came very quickly for me.
"As a driver, you put most of the pressure on yourself, so it's about focusing on what's important and taking the car around the track as fast as you can," Hartley, who is 28 next month, added.
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