Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE said Thursday that Russia's involvement in Ukraine remained the major barrier to warmer relations between Washington and Moscow.

Tillerson, who met with representatives to the Organization for Security and Cooperation in Europe (OSCE) on Thursday, said that the Trump administration made clear to the Kremlin from the beginning that settling tensions in Ukraine would be a necessary to forging closer ties between the U.S. and Russia.

"The issue that stands in the way is Ukraine,” Tillerson said.

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"We can have differences in other arenas, in Syria, we can have differences in other areas," he said. "But when one country invades another, that is a difference that is hard to look past or to reconcile."

The U.S. has decried Moscow's military involvement in eastern Ukraine, where it has backed pro-Russia separatists, and its 2014 annexation of Crimea from Ukraine.

In remarks at an OSCE session, Tillerson said that the U.S. would "never accept Russia’s occupation and attempted annexation of Crimea" and warned that sanctions related to the matter would remain in place until Moscow returns control of the peninsula to Ukraine.

Tillerson made no mention on Thursday of Russia's efforts to meddle in the United States' 2016 presidential election, though he hinted at such activities a day earlier in Brussels, saying that Moscow's efforts to "undermine Western institutions" remained an obstacle to warmer ties with NATO.

"We have been clear with Russia that we cannot return to business as usual in the NATO-Russia relations, as long as Russia continues its illegal occupation of Ukraine," he said at a press conference at NATO headquarters.

"And Russia’s continued use of hybrid warfare seeks to undermine Western institutions, and this stands as a significant obstacle to normalizing our relations, as well."

--This report was updated at 12:12 p.m.