Stoltenberg, speaking at a joint press conference in Warsaw with Poland's right-wing Prime Minister Beata Szydlo, urged Moscow to respect international conventions on keeping other countries informed of the manoeuvres.
Zapad 2017 ("West"), has stoked alarm in NATO members Poland and the Baltic states, all former members of the Soviet bloc.
It takes place in Belarus, which border three NATO member states, and comes as a more assertive Russia pushes back against what it sees as the alliance's unjustified expansion into eastern Europe.
The Vienna Document requires signatory nations to provide advance information of exercises and to allow observer teams to avoid any dangerous misunderstandings.
Stoltenberg was even more outspoken on Thursday in Italy when he said that "the aggressive behaviour of Russia has undermined stability and security in Europe".
Today, he vowed 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.
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.
According to NATO, Belarus has invited military liaison missions to attend a special visitors day on its territory, with two alliance experts due to attend.
Russia's Interfax news agency reported this week that the Belarussian defence ministry had invited observers from seven countries: Ukraine, Poland, Lithuania, Latvia, Estonia, Sweden, and Norway.
Russia has dismissed the concerns over the exercises. "I do not see any reason to be afraid," deputy defence minister Alexander Fomin told the Rossiya 24 news channel. "Everything, as usual, will be open and friendly."
Later, Stoltenberg visited a US-led NATO battalion based in the northeastern Polish town of Orzysz.
The German and Lithuanian presidents meanwhile visited a similar NATO base some 250 kilometres away in Rukla, Lithuania.
NATO deployed four multinational units to Poland and the Baltic states of Estonia, Latvia and Lithuania to reassure its easternmost allies unsettled by Russia's frequent military exercises near the region following its 2014 annexation of Crimea from Ukraine.
The Orzysz and Rukla NATO bases are both close to Russia's highly militarised Kaliningrad exclave and the Suwalki Gap, a strategically important land corridor critical to the security of the Baltic states.
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