Is Russia trying to hide its massive military exercise from NATO?

Russia has been accused by the head of NATO of blocking the alliance from properly observing next week's "Zapad" military exercises, when about 100,000 Russian troops are expected to mobilize on the EU's eastern borders.

Jens Stoltenberg, NATO's secretary general, said an offer from Russia and Belarus for three of their experts to attend some aspects of the huge exercise fell short of the Kremlin's international obligations.

"Briefings on the exercise scenario and progress; opportunities to talk to individual soldiers; and overflights over the exercise. This is something that is part of the Vienna document, an agreement regulating transparency and predictability relating to military exercises," said Stoltenberg, during a visit to an Estonian military base, where British troops have been stationed since March.

"So we call on Russia to observe the letter and the spirit of the Vienna document. Transparency and predictability are even more important when tensions are high to reduce the risks of misunderstandings and incidents. NATO remains calm and vigilant and we are going to keep Estonia and our allies safe."

Under Organization for Security and Cooperation in Europe rules in the Vienna document, nations conducting exercises involving more than 13,000 troops must notify other countries in advance and be open to observers.

Russia and Belarus claim the Zapad ("west") exercises, which will be held in Belarus and parts of western Russia between Sept. 14-20 will involve about 12,700 troops.

NATO, however, believes many more troops are set to be involved. The prime minister of Estonia, Jüri Ratas, who joined Stoltenberg at the base in Tapa, about 75 miles (120km) from the Russian border, confirmed that his government believed about 100,000 Russian soldiers would be mobilized during the exercise.

He said, "I would like to say that we are concerned about the nature and lack of transparency of the exercise. Our attitude remains cool and confident. Along with our allies, we will monitor the exercise very closely and remain ready for every situation."

There has been speculation that Russia could use the upcoming exercises as a cover for the permanent movement of troops and equipment into Belarus or even an offensive against NATO states, something Moscow has adamantly denied.

Russia claims that that western concerns about Zapad are unfounded, saying the war games will be purely defensive and are designed to help practice dealing with a terrorist threat in the future. The country holds the Zapad exercises every four years.

Tensions following the illegal annexation of Crimea by Russia in 2014 have, however, seen the establishment of four multinational battle groups in three Baltic states – Estonia, Latvia, and Lithuania – as well as Poland, amounting to approximately 4,500 troops, including those from the UK.

Stoltenberg, a former prime minister of Norway, told reporters in Estonia, "We are not changing our military posture because of the Zapad exercise, but NATO has only implemented important changes in our military posture in response to a more assertive Russia as seen developing in recent years, with more Russian troops close to our borders, more Russian equipment, and more exercises. And not least, of course, the use of military force against a neighbor, Ukraine."

He added that while he saw no imminent threat to NATO states, that the battle groups' presence sent a strong message "that an attack on one ally will trigger a response from the whole alliance."