VILNIUS (Reuters) - The U.S. Army’s top European commander on Wednesday called on Russia to open its major military exercise later this year to observers to assuage the anxieties of its neighbors.

Russia has unveiled plans to stage its Zapad 2017 exercise near its western borders this autumn but has not said how many troops will take part.

Lithuanian president Dalia Grybauskaite said the Baltic countries were concerned about the exercise and called on NATO for additional security measures.

“For me, Zapad represents an opportunity for the Russians to demonstrate that they are committed to security and stability in Europe also through transparency, by inviting media, by inviting observers, more than the minimums required by Vienna, to demonstrate, to show what’s going on, to be transparent,” U.S. lieutenant general Ben Hodges told reporters in Vilnius.

“That would be the first and most important thing,” he added. “I think that would lower some anxiety”.

The Cold War-era treaty known as the Vienna document sets out rules for large-scale exercises and other military activity.

The U.S. forces’ response to Russia’s military exercise “will depend on what our leadership says is the appropriate posture”, Hodges said.

He added that the U.S. was also looking at ways to address Russia’s use of unmanned drones, such as those deployed in Eastern Ukraine and Syria.

The Baltic states, annexed by the Soviet Union in the 1940s but now part of NATO and the European Union, have felt vulnerable since Moscow’s annexation of Crimea and its participation in military conflicts in Eastern Ukraine.

The U.S. has deployed an armored brigade and an aviation brigade to Europe this year, originally ordered by former U.S. president Barack Obama as part of NATO efforts to deter Russia from further meddling in Eastern Europe.

As part of the deterrence effort, NATO nations are sending four battle groups into the Baltic States and Poland, some of them deployed in the Suwalki gap between Belarus and Russia, the sole land connection between the Baltics and the rest of NATO.

“This is the first U.S. brigade of a series that will continue for as long as necessary, I assume at least the next five years,” said Hodges. “Heel-to-toe, brigade after brigade after brigade, no gaps.”