The Kremlin views the continued strain on Russia's relations with the United States as a major disappointment of the year, President Vladimir Putin's spokesman said Friday.

Ties between Moscow and Washington sank to a post-Cold War low following Russia's annexation of Crimea in 2014 and the allegations of its meddling in the U.S. presidential election last year. The Kremlin's hopes for warmer dealings under President Donald Trump have withered amid the Congressional and FBI investigations into alleged links between Trump's campaign and Russia.

The two countries introduced tit-for-tat measures to rebuke each other all year, ranging from restrictions on embassy staff to legislation targeting state-owned media.

Asked about the Kremlin's biggest disappointments of 2017, Putin spokesman Dmitry Peskov told reporters that worsening relations with the U.S. were "certainly" on the list.

Russia has denied any interference in the U.S. election, and Peskov noted that the Kremlin is watching the ongoing U.S. investigations with "bewilderment."

"It's the U.S. internal business, but it certainly hurts bilateral relations and we regret that," he said.

Peskov reiterated the Kremlin's position that Russia seeks good relations with the U.S. based on "mutual trust and mutual respect," but added that "it takes two to tango."

Russian officials and lawmakers last week expressed dismay with the U.S. decision to provide Ukraine with lethal weapons, including anti-tank missiles, warning the move would only fuel hostilities in eastern Ukraine.

Speaking to reporters at the Pentagon Friday, Defense Secretary Jim Mattis said the U.S. approach to Ukraine's conflict with Russia-backed separatist rebels wasn't changing despite the decision to supply weapons to Ukraine.

"I don't see an evolving U.S military role in Ukraine," Mattis said. "Right now, we have some trainers there helping to train their army to NATO standards, and that has a lot to do with making certain it serves the needs of the Ukrainian people, in the way democracies' armies do, so the U.S. military role remains the same."

He said there was no plan to expand the U.S. footprint in Ukraine and also downplayed the potential effect of the new arms his country is supplying.

"As long as no one wants to invade Ukraine, hopefully it won't have any big impact," Mattis said. "They're defensive weapons."