Mattis said that the U.S. decision to supply "defensive" weapons to Ukraine should be no cause of alarm for the Russians.
Ukraine and Russian-backed rebels Wednesday began to exchange more than 300 prisoners in the war-torn east of the country.
The new arms include Javelin anti-tank missiles to fight against separatists armed with Russian tanks.
The DoD said any decisions on the sale of anti-tank and anti-aircraft weapons would come from the State Department.
The annual Rapid Trident military exercises involve an "unprecedented" number of 2,500 soldiers from 15 countries.
Russia asked the UN Security Council to authorize the deployment of a lightly-armed mission to protect OSCE observers.
Defense Secretary Jim Mattis acknowledged "problems" plaguing the United States in off-the-cuff remarks to deployed troops.
From Ukraine to the Baltics, troops will be maneuvering on both sides of the line that divides Russia from NATO and its partn...
Defense Secretary Jim Mattis on Thursday pledged continued aid to Ukraine as it clashes with pro-separatist, Russian-backed f...
Mattis' trip comes as the United States is mulling giving Ukraine lethal weapons after Russia's annexation of Crimea.