The Estonian Ministry of Defense said the country is ready to help Lithuania if Russian troops in Belarus threaten its security.
Russia has moved thousands of troops into Belarus as part of its efforts to pressure Ukraine in recent months, which could threaten Lithuania's security.
Secretary General of the Ministry of Defense Kusti Salm said: "Today, an unprecedented number of Russian troops have gathered in Belarus, there have never been so many - there are at least 15,000 of them there today and they are still being brought in."
He said joint Russian-Belarusian drills have already begun and the whole spectrum of Russia's land and air weapons have been moved to the country.
"In Lithuania, there is definitely a lot of anxiety because military practice operations are being held with weapons and soldiers that are very close to their border. One can only guess exactly where they want to go, what their ultimate goal is," Salm said.
Asked how much of a security risk the Russian troops in Belarus pose to Lithuania, General Martin Herem said it is the same threat as Russian troops pose to Estonia in Pskov, Petseri and elsewhere behind Estonia's eastern border.
"Of course, the political environment today does not favor this [an attack], they are unlikely to cross the border," he said, but added if the troops stay in Belarus, they are much closer to Lithuania and Poland than before.
"[A]nd that, of course, causes all kinds of tensions, Herem said. "And it's the opposite of what Russia is saying, that the United States has brought its forces close to its borders."
Asked if Estonia would offer help to Lithuania if needed, Salm said: "We have already helped Lithuania through the summer hybrid [migrant] crisis and, of course, we will be the first to help our closest allies."
At the same time, he emphasized that there is currently no direct military threat to Lithuania but Estonia is watching the situation and for misinformation.
Editor: Helen Wright