His comments come shortly after the U.S. and Ukraine signed a minerals deal, an agreement that Vance said showed the White House is making progress.

Ukraine's Air Force lost a Su-27 fighter jet during a Russian aerial attack on April 28. The aircraft was conducting a combat mission, supporting ground troops. The loss follows the earlier downing of the second F-16 fighter jet, which reportedly occurred in the Russia-bordering northeastern region of Sumy on April 16.

It is "going to be up to the Russians and Ukrainians now that each side knows what the other's terms for peace are. It's going to be up to them to come to an agreement and stop this brutal, brutal conflict," Vance told Fox News' Bret Baier.