"Hopefully, it won't have an impact" on the tense relations between Ukraine and Moscow over Russia's support for separatists in eastern Ukraine, Mattis said. "They're defensive weapons," Mattis said of the arms for Ukraine that reportedly include Javelins and anti-aircraft weapons.
In an informal year-end session with Pentagon reporters, Mattis also said there were no intentions to increase the small U.S. troop presence in Ukraine, where U.S. troops have been training Ukraine forces well away from the frontlines in eastern Ukraine.
"As you know, we have some trainers there -- training to NATO standards," Mattis said. "There's not plan to change that number at all -- as long as no one wants to invade Ukraine. I don't see any evolving U.S. military role in Ukraine right now."
The provision of defensive weapons to Ukraine has long been backed by the Defense Department, but the Obama administration withheld approval and the Trump administration also initially was reluctant to sanction a deal that could antagonize Russia.
Earlier last week, the Trump administration announced that it would permit the sale by U.S. manufacturers to Ukraine of sniper rifles, scopes and ammunition.
Last Saturday, the State Department made the announcement that the U.S. would go ahead with the provision of heavier defensive weapons to Ukraine.
Immediately following the State Department announcement, Russian Deputy Foreign Minister Sergey Ryabkov said the U.S decision to supply Ukraine with anti-tank weapons was "clearly pushing [Ukraine] to new bloodshed."
According to the United Nations, more than 10,000 have been killed in fighting in Ukraine since the separatists in 2014 declared the independence of the Donetsk and Luhansk regions of eastern Ukraine.
Despite the tensions between Kiev and Moscow over the arms deal, the Russian-backed separatists and Ukraine agreed Monday to their first prisoner swap in 14 months.
Russian state media reported that Ukraine had agreed to release 306 prisoners in exchange for 74 being held by separatists.
-- Richard Sisk can be reached at Richard.Sisk@Mililtary.com.