"The Russians have not given us a lot of reason to trust the numbers that they say. But again, the exercise hasn't happened yet, so we don't know what they are going to do," Lieutenant General Ben Hodges said at a joint news conference with Lithuanian Defense Minister Raimundas Karoblis in Vilnius.
Moscow says that Zapad 2017 will involve fewer than 13,000 troops, but NATO officials think that the number was artificially reduced by splitting the exercise into separate parts so as not to give wider access to observers, as required by international rules.
The war games will be held in the western part of Russia and its Baltic Sea exclave of Kaliningrad and in Belarus between Sept. 14 and 20.
Hodges said that Russia could dispel concerns by inviting journalists to observe the drills the way Western countries do.
"I think this would all be solved and everybody would be relaxed if the Russian Federation invited the media to everything that they do the way that Lithuanian armed forces do, the German armed forces do, that American armed forces do," the US general said.
"If the Russian Federation is truly interested in stability and security, then be transparent, invite media to see everything that they do. In the West, we have journalists, think-tanks, parliamentarians, civic organizations, they come to every exercise that we do, so that people don't have to be concerned or worried about what we are actually doing," Hodges said.
"So what is the risk could be alleviated with transparency. We have Russian inspectors come to our exercises all the time. I've had Russians visit me in Germany, Poland, Bulgaria during exercises. I would love to have them follow me around all day just to lower the temperature and to build some confidence," he said.
The general said that he feels confident in the deterrence capability NATO has created in the region, but added that the Alliance has to remain vigilant.
"But we also have to always remain vigilant, to continue to try to understand what potential adversaries are doing. Of course, the Alliance faces several different types of threats. Being alert is the responsibility for all of our security services," Hodges said.
"But I don't have a good record for predicting the future", he added.
The US has sent extra fighter jets and a company of troops to Lithuania for the period of Zapad, but the US general said that the Alliance's exercises will be held according to schedule.
Also, some 1,000 troops from different NATO countries were earlier this year deployed to Lithuania, Latvia, Estonia and Poland each
Russia and Belarus say that Zapad 2017 will be "purely defensive". Lithuanian Defense Minister Karoblis said on Friday that "defense may be followed by a counteroffensive and a simulation of an offensive".