Russia and Belarus say the Zapad 2017 exercise, which runs from September 14-20 not far from the borders of NATO allies Estonia, Latvia, Lithuania and Poland, will involve less than 13,000 troops, but NATO believes that number will actually be into six figures. Under international rules, the two should allow wider access to monitors if troop numbers exceed that figure.

NATO officials said there will be three experts on mission to observe military exercises between Russia and Belarus, but alliance Secretary General Jens Stoltenberg wants the two countries to allow broader monitoring.

NATO spokesperson Oana Lungescu said the three experts will attend so-called Visitors’ Days in Belarus and Russia after they were invited to attend.

But she said international rules permit monitors to have much wider access, including briefings on the exercise, opportunities to talk to soldiers and overflights.