As reported by Censor.NET citing AFP via YahooNews, Mattis's remarks come amid heightened tensions with Russia, which is preparing for massive military exercises in its western military region in September, and has deployed a missile system in neighbouring Kaliningrad.
"Any buildup of Russian combat power in an area where they know, and we all know, they are not threatened by anything that we are doing... is simply destabilising," Mattis said at a press conference with Lithuanian President Dalia Grybauskaite.
He however refused to discuss whether Vilnius had requested a permanent Patriot missile deployment.
"We will make those decisions in consultation with the Lithuania government," he said.
The Patriot is a mobile air defence system made by Raytheon designed to intercept tactical ballistic missiles, low-flying cruise missiles and aircraft.
"Everyone knows this is not an offensive capability," Mattis said when asked about regional air defences.
"We will deploy whatever capability is necessary here," Mattis added.