Warsaw (Reuters): NATO Secretary General Jens Stoltenberg said on Friday the alliance would closely watch Russian military exercises in western Russia and Belarus next month, urging Moscow to be transparent about the drills .

The manoeuvres, the largest in years, with tanks, naval and air units operating in and around the Baltic and North Sea, have raised NATO’s concern that the official number of troops participating might be understated.

“We are going to be watching very closely the course of these exercises,” Stoltenberg told reporters after meeting Polish Prime Minister Beata Szydlo on a visit to check on the deployment of the U.S.-led alliance’s forces in the country’s east.

“All countries have the right to exercises of their armed forces, but the countries should also respect the obligation to be transparent .”

Russia said that 13,000 troops would participate in the Sept. 14-20 drills , which under an international agreement is the limit for not requiring the presence of external observers. Western estimates have put the number of troops involved much higher.

Stoltenberg will meet with Polish, Turkish and Romanian foreign ministers later on Friday before visiting NATO troops in Poland’s Orzysz, about 57 km (35 miles) south of Russia’s Baltic Sea enclave of Kaliningrad, where Moscow has stationed nuclear-capable missiles and an S-400 air missile defence system.

“(The NATO deployment) is a clear signal that an attack on one ally is an attack on the whole alliance,” Stoltenberg said. “The matter here is to prevent conflicts and not to provoke them.”