PABRADE TRAINING AREA, Lithuania (Reuters) - U.S. Defense Secretary Jim Mattis said on Wednesday that a Russian missile deployment near the Baltic states was "destabilising", and officials suggested the United States could deploy a Patriot missile battery in the region for NATO exercises in the summer.
U.S. allies are jittery ahead of war games by Russia and Belarus in September that could involve up to 100,000 troops and include nuclear weapons training -- the biggest such exercise since 2013.
The drills could see Russian troops near the borders of Poland, Lithuania, Latvia and Estonia.
Russia has also deployed Iskander missiles in Kaliningrad, its enclave on the Baltic Sea. It said the deployment was part of routine drills, but U.S. officials worry that it may represent a permanent upgrade.
Asked during a trip to Lithuania about the deployment, Mattis told a news conference: "Any kind of build-up like that is simply destabilising."
The United States is ruling out any direct response to the Russian drills or the Iskander deployment.
But at the same time, U.S. officials, speaking on condition of anonymity, raised the possibility that a Patriot missile battery could be deployed briefly to the Baltic region during NATO exercises in July that focus on air defence, known as Tobruk Legacy.
The officials stressed that the Patriots, if deployed, would be withdrawn when the exercises were over. That would most likely happen before the Russian drills began, they said.
Mattis declined to comment directly on the possible Patriot deployment to reporters after talks in Vilnius.
"The specific systems that we bring are those that we determine necessary," Mattis said, saying that NATO capabilities in the region were purely defensive.
It was Mattis's first trip to the Baltic states, which fear Russia could attack them in the same way that it annexed Ukraine's Crimean peninsula in 2014. The states are concerned about their lack of air defences and considering upgrading their military hardware.
Asked about Baltic air defences on a visit to the Pabrade training ground, Mattis told reporters:
"We will talk to the leaders of each of the nations, and we will work this out in Brussels and we will work together if necessary.
"The reason for the deployment you see right now is the lack of respect for international law by a nation in the region, and so long as the nation shows respect, we would not have to deploy that," Mattis told reporters, standing in front of a German Leopard tank.
A German-led battalion was deployed to Lithuania this year as part of a NATO effort to deter any Russian aggression.
Asked about any future Patriot deployment, Lithuania's President Dalia Grybauskaite, standing next to Mattis, said: "We need all necessary means for defence and for deterrence, and that's what we'll decide together."
The scale of this year's Russian "Zapad" ("West") manoeuvres, which date from Soviet times, when they were first used to test new weapon systems, is one of NATO's most pressing concerns. Western diplomats say the exercises pose an unusual threat.
But Mattis told reporters: "It's a routine exercise. I trust it will stay routine."
Estonian Defence Minister Margus Tsahkna told Reuters last month that NATO governments had intelligence suggesting Moscow may leave Russian soldiers in Belarus once the Zapad 2017 exercises are over, also pointing to public data of Russian railway traffic to Belarus.
Moscow denies any plans to threaten NATO and says it is the U.S.-led alliance that is undermining stability in eastern Europe. It has not said how many troops will take part in Zapad 2017.
Additional reporting by Andrew Osborn in Moscow; Editing by Angus MacSwan and Kevin Liffey