“We have communicated to the Russian government that what happens to Mr. Navalny in their custody is their responsibility and they will be held accountable by the international community,” Sullivan told CNN.

“In terms of the specific measures that we would undertake, we are looking at a variety of different costs that we would impose and I am not going to telegraph that publicly at this point, but we have communicated that there will be consequences if Mr. Navalny dies,” he added.

Reuters reported earlier that Navalny is facing an increasing risk of kidney failure and his vision is deteriorating after more than two weeks on hunger strike.

Navalny, 44, a prominent opponent of Russian President Vladimir Putin, started refusing food on March 31 to protest a lack of proper medical treatment for his leg and back pain.