NATO Secretary General Jens Stoltenberg on Friday urged the Kremlin to comply with rules on transparency as it gears up for huge military exercises along the alliance's eastern flank next month.
The drill, named Zapad 2017 ("West"), has stoked fresh alarm in NATO-members Poland and the Baltic states as a more assertive Russia pushes back against what it sees as the alliance's unjustified expansion into eastern Europe.
"I call on Russia to ensure compliance with its obligations under the OSCE Vienna Document, because predictability, transparency is especially important when we have increased military activity along our borders," Stoltenberg told reporters in Warsaw at a joint press conference with Poland's right-wing Prime Minister Beata Szydlo.
The Vienna Document requires all sides to provide advance information about exercises and allow observer teams so as to avoid any dangerous misunderstandings.
The NATO chief had much stronger words for Moscow on Thursday in Italy when he said that "the aggressive behaviour of Russia has undermined stability and security in Europe."
He vowed on Friday that the alliance would "be watching very closely as this (Zapad) exercise takes place next month" in Belarus, which borders alliance members Latvia, Lithuania and Poland.
Under Vienna Document provisions, manoeuvres involving more than 13,000 troops must be notified in advance and be open to observers.
Belarus has said Zapad 2017 involves 12,700 troops, just under the limit, but Lithuania and other critics claim there could be as many as 100,000.
'Centre of gravity'
According to NATO, Belarus has invited military liaison missions to attend a special visitors day on its territory, with two alliance experts due to go along.
Meanwhile, Russia's Interfax news agency reported this week that the Belarusian defence ministry had invited "observers from seven countries, namely Ukraine, Poland, Lithuania, Latvia, Estonia, Sweden, and Norway ... to this (Zapad 2017) event."
Russia has dismissed concerns over the exercises, with Russian Deputy Defence Minister Alexander Fomin telling the Rossiya 24 news channel that "I do not see any reason to be afraid. Everything, as usual, will be open and friendly."
Stoltenberg was due to visit later Friday a US-led NATO battalion based in the northeastern Polish town of Orzysz.
The multinational unit is one of four deployed by NATO this spring to Poland and the Baltic states of Estonia, Latvia and Lithuania in order to reassure its easternmost allies unsettled by Russia's frequent military exercises near the region in the wake of its 2014 annexation of Crimea from Ukraine.
General Ben Hodges, commander of US ground forces in Europe, said last week that "Poland has become for the United States Army the centre of gravity for everything that we're doing in terms of deterrence" regarding Russia.
Aside from leading the NATO force in Orzysz, the US Army set up a new European headquarters in Poland in May to command some 6,000 of its troops deployed in NATO and Pentagon operations across the alliance's eastern flank since the beginning of the year.