U.S. Secretary of State Rex Tillerson has said the biggest problem preventing a warming of U.S.-Russian relations is Russia's aggression in Ukraine.
Speaking at a meeting with European foreign ministers in Vienna on December 7, Tillerson said the United States will not lift sanctions against Russia until it pulls its forces out of eastern Ukraine and Crimea.
"The issue that stands in the way is Ukraine," Tillerson said at a news conference after the Organization for Security and Cooperation in Europe (OSCE) ministers meeting.
"We can have differences in other arenas.... But when one country invades another, that is a difference that is hard to look past or to reconcile," he said.
"And we've made this clear to Russia from the very beginning: That we must address Ukraine, it stands as the single most difficult obstacle to us renormalizing relations."
After Tillerson's strong statement on Ukraine, a face-to-face meeting with his Russian counterpart appeared to produced no breakthroughs on points of tension between the two countries.
"I'm not going to tell you specifically what we get. We get progress, that's what we get," Tillerson said in the Austrian capital on December 7 following his talks with Russian Foreign Minister Sergei Lavrov.
"We get dialogue, we get cooperation, we don't have it solved. You don't solve it in one meeting," he added.
The United States and European Union are pushing Moscow to allow a robust United Nations peacekeeping force to be deployed to eastern Ukraine, where Russia-backed separatist militia are fighting government forces in a conflict that has killed more than 10,000 people since it started in April 2014.
Lavrov said he told Tillerson that Moscow was "concerned about attempts to turn our initiative about a deployment of UN guards to ensure OSCE [Special Monitoring] Mission's security upside-down."
Conflict In Ukraine
Moscow and Washington disagree over the mandate that the proposed UN force would have.
The Kremlin says the UN mission should have the powers to protect OSCE monitors in eastern Ukraine.
But Western powers fear President Vladimir Putin wants to limit the force’s mandate in a way that any cease-fire would merely consolidate the gains of Russia-backed separatists.
Tillerson has said Washington wants the proposed UN force to have additional peacekeeping powers. That could include the authority to disarm Russia-backed separatist militias in eastern Ukraine.
In his speech at the OSCE foreign ministers meeting, Tillerson said on December 7 that the United States "will never accept Russia's occupation and attempted annexation of Crimea."
He said U.S. sanctions that were imposed after Moscow's illegal annexation of Crimea in March 2014 "will remain in place until Russia returns full control of the peninsula to Ukraine."
"In eastern Ukraine, we join our European partners in maintaining sanctions until Russia withdraws its forces from [separatist-held parts of the Donetsk and Luhansk regions] and meets its Minsk commitments," Tillerson added.
Several cease-fire deals announced as part of the Minsk accords -- September 2014 and February 2015 pacts aimed at resolving the conflict -- have failed to hold.
Referring to cease-fire violations in eastern Ukraine, Tillerson said, "We should be clear about the source of this violence."
"Russia is arming, leading, training, and fighting alongside antigovernment forces. We call on Russia and its proxies to end its harassment, intimidation, and its attacks on the OSCE Special Monitoring Mission," he also said.
WATCH: U.S. Secretary of State Rex Tillerson said sanctions against Moscow will remain in place until Russia withdraws from Ukraine. (Reuters)
Lavrov told the OSCE meeting that "all the responsibility is with Ukraine" as far as violence in the east was concerned.
The Russian foreign minister also said that Russia supported the OSCE settlement plan and its Special Monitoring Mission in eastern Ukraine.
"Both OSCE functions should be aimed at direct dialogue between Kyiv, Donetsk, and Luhansk as stipulated in the Minsk package of measures to which there is no alternative," Lavrov said.
Ahead of the meeting between Tillerson and Lavrov, Tillerson met on the sidelines of the OSCE summit with Ukrainian Foreign Minister Pavlo Klimkin.
Klimkin told Russia's Interfax news agency that they discussed "joint work on the UN peacekeeping mission" in eastern Ukraine, "energy security," and "U.S. support for an efficient and independent anticorruption system in Ukraine."
Klimkin also held talks with Lavrov late in the day for what was described as their first face-to-face meeting since the Ukrainian minister took office in June 2014.
Ukraine's foreign minister said in a Twitter message that an exchange of prisoners between the separatists and Kyiv was discussed at the meeting.
"We should pull it off in the coming weeks because it's about people and their loved ones," Klimkin wrote.
On December 6, a day before the start of the two-day Vienna conference, Tillerson said at NATO headquarters in Brussels that "Russia’s aggression in Ukraine remains the biggest threat to European security."
"We prioritize ending the violence," Tillerson said. "That’s our first priority, and to seek to do that we need to put a peacekeeping force in place."
The OSCE has deployed 600 unarmed monitors in eastern Ukraine to investigate and discourage cease-fire violations. They often come under fire from the warring factions.