Lithuanian intelligence services say Moscow’s speed could leave Nato with no option other than to have military forces deployed in the region ready for action.
With Russia upgrading its military in the Kalingrad region, intelligence suggests Nato reinforcements could potentially be prevented if there was an attack.
Lithuania, Latvia and Estonia have been increasingly nervous since the Russian takeover of Crimea in 2014.
According to the Lithuanian intelligence service’s annual threat assessment Russia’s military strength could reduce lead times for any attack.
The Russian upgrade included Su-30 fighter aircraft and missile systems allowing ships to be targeted almost anywhere in the Baltic Sea.
Lithuanian Defence Minister Raimundas Karoblis said: “This is a signal to Nato to improve its decision speed.
“Nato’s reaction time is not as fast as we would like it to be.”
This year Nato is deploying 1,000 soldiers in each of the Baltic states and Poland in addition to smaller US forces already in the region.
Mr Karoblis said: “The force is adequate in the short-term, but in the medium-term perspective we would like more capability, and not only land troops but also air defences and capabilities to counter any blockade.”
The intelligence report said Russia is monitoring and suppressing radio frequencies used by NATO pilots over the Baltic Sea.
Russia is also using commercial and scientific ships for surveillance, the report said.
Intelligence officers said disinformation aimed at discrediting Nato soldiers stationed in Lithuania, such as a recent false report of a rape by German soldiers, was likely to persist.
Remigijus Baltrenas, head of Lithuanian military counterintelligence, said: “Provocations against NATO units in Lithuania will continue and will get bigger.”
But Russia dismissed Lithuania’s fears saying the intelligence report is a display of anti-Russian sentiment.
Kremlin spokesman Dmitry Peskov said: “There is total Russophobia, hysterical Russophobia going on.
“Moscow has always supported good relations with the Baltic states.”