By Andrea Shalal

BERLIN (Reuters) - Lithuania's ambassador to Germany on Monday urged Germany's next coalition government to make good its promise to increase defence spending, warning that Russia could prey on any signs of weakness in the NATO alliance.

Angela Merkel secured a fourth term as German chancellor but her conservative bloc lost significant ground in Sunday's election and will now negotiate an untested three-way coalition with the pro-business Free Democrats and the environmental Greens, who have historically opposed big increases in military spending.

Darius Semaska, Lithuanian envoy in Berlin, told Reuters his country valued Germany's role in leading a NATO force in Lithuania, but said Berlin also needed to add "real capacities" and boost defence spending as agreed by NATO members.

NATO members agreed in 2014 to move toward spending 2 percent of gross domestic product on defence over the next decade. NATO data shows that Germany is due to spend 39.5 billion euros (34.77 billion pounds) or 1.2 percent of GDP on the military in 2017.

"We very much look forward to this policy line being continued, and that it is strengthened by real capacities ... to show the potential aggressor that there is capacity to defend us," he said.

Failure to boost spending could embolden a Russian "adventure," he said, citing parallels to recent Russian actions and its behaviour before the annexation of Ukraine's Crimea region.

Semaska said Lithuania valued the Greens' strong "valued-based" line against Russia over its annexation of the Crimea region of Ukraine and its support for separatists in eastern Ukraine, but noted that "tactics" were also important.

Lithuania has expressed major concern over the Zapad military exercises Russia recently held in its western region. Lithuanian President Dalia Grybauskaite made the wargames the centrepiece of her annual speech to the United Nations General Assembly in New York earlier this month.

NATO said it was worried Moscow could leave behind forces after the war games for a possible occupation of Western neighbours.

Russia and Belarus have said the military exercises were purely defensive in nature.

(Reporting by Andrea Shalal; Editing by Raissa Kasolowsky)