Lithuanian President Dalia Grybauskaite arrives at a EU leaders summit in Brussels, Belgium, March 19, 2015. Thomson Reuters

The Lithuanian president has said she wants an ongoing US troop presence in her country in light of increased Russian activity in the region and in response to Moscow's preparations for military exercises in neighboring Belarus late this year.

"We need the serious involvement of the US to not only deter but to defend," President Dalia Grybauskaite told The Wall Street Journal on Tuesday, after a meeting with US Defense Secretary James Mattis. "It is important to have adequate response capabilities against possible threats."

Planned war games by Russia and Belarus slated for September could involve up to 100,000 troops and include nuclear-weapons training. Mattis has criticized the buildup ahead of the exercise, as well as Russia's deployment of missiles to Kaliningrad — its semi-enclave on the Baltic Sea.

"Any kind of buildup like that is simply destabilizing," Mattis said. Moscow said the missiles were part of routine drills, but US officials worried it was a permanent upgrade to Kaliningrad's missile capability.

The US has had 150 soldiers in Lithuania since 2014 but now plans to station soldiers from a heavy tank brigade in the country temporarily, according to The Journal. That rotation will begin in June with soldiers taking part in a multinational exercise.

NATO soldiers attend a NATO military exercise at Raposa beach, near Setubal, Portugal, October 20, 2015. Rafael Marchante/REUTERS

"Our goal is that not only NATO troops need to be involved in deterrence in the Baltic region, but also, bilaterally, US troops,” Grybauskaite told The Journal, saying that the US would likely be the first to respond in the event of a confrontation with Russia, as NATO decision-making did not move quickly.

"Having in mind the challenges we are facing and the increase in tensions in our region it would be preferable to have the US on a permanent basis," she said.

Raimundas Karoblis, Lithuania's defense minister, has criticized NATO's reaction speed in the past and this week echoed the president's remarks. "The presence of Americans is a multiplying factor for deterrence," he told The Journal. "With Americans here it is a game changer."

The Baltic states have sounded alarm about Russia activity in Eastern Europe for several months.

Russian President Vladimir Putin. Aleksey Nikolskyi/Reuters

In April, Karoblis told The Guardian that his country was "taking very seriously" propaganda efforts organized by Moscow meant to destabilize the region.

"Russia is a threat," he said at the time. "They are saying our capital Vilnius should not belong to Lithuania because between the first and second world wars it was occupied by Poland."

"There are real parallels with Crimea's annexation [from Ukraine]," he added. "We are speaking of a danger to the territorial integrity of Lithuania."

Estonia's national intelligence report for 2016 said the "Baltic Sea area is especially vulnerable to threats from Russia" and that Russian intelligence agencies were conducting operations to influence the Estonian defense forces and public.

In recent months, NATO countries have stepped up their deployments to Eastern Europe, which is already bristling with offensive and defensive weaponry.

In Lithuania, a 1,000-member NATO force of troops from Germany, Belgium, Luxembourg, the Netherlands, and Norway is already on site. Battle groups from the UK and Canada are also headed to Poland, Latvia, and Estonia.

More than 1,100 troops — 900 of them from the US — have taken position in Poland. "This is a mission, not a cycle of training events," said US Army Lt. Col. Steven Gventer, who heads the battle group in Poland. "The purpose is to deter aggression in the Baltics and in Poland ... We are fully ready to be lethal."

US soldiers arrive in Zagan, Poland, January 12, 2017, as part of NATO deployment. REUTERS/Zbigniew Janicki/Agencja Gazeta

The US has deployed dozens of helicopters and thousands of pieces of military equipment to Germany in recent weeks to underscore Washington's "rock-solid commitment to Europe." Some of the US tanks recently deployed to the region have changed their camouflage scheme to better blend in with the environment.

US officials have said a long-range Patriot missile battery — purely for defensive use — could move into the Baltic region this year as part of planned military exercises but would be gone by the time the Russia-Belarus military exercise started.

Poland also plans to buy $7.6 billion worth of Patriot air-defense missiles to counter Moscow, which has been greeted with Russian ire.

A Patriot missile battery. Flickr / D'oh Boy

There have been several encounters between NATO forces and their Russian counterparts in the Baltic Sea region in recent months.

French jets shadowed Russian fighters over the region earlier this year, in international airspace along NATO's northern border. Russian aircraft also did flybys of a US destroyer in February, which the US Navy called an unsafe maneuver.

This week, a Russian fighter flew within 20 feet of a Navy patrol craft — though the Navy called that encounter "safe and professional."

The Lithuanian president, like her counterpart in Estonia, has backed US President Donald Trump's calls for NATO member states increase their defense spending.

"Security of the eastern border of NATO is the security of all of NATO," Grybauskaite told The Journal. "If we fail in any of the 28 members, it will be a failure of all of NATO."