Duda made the remark at a joint news conference with the NATO chief, Poland's PAP news agency reported.
“There is no doubt in my mind that we need reinforcements here, on the eastern flank of the North Atlantic Alliance,” the Polish president said.
He added that Russia was preparing to stage joint exercises with Belarus later this month, and had already moved troops and military hardware from its Far Eastern regions for the purpose.
“Everyone can see what is going on - Russia is increasing its military presence in our part of Europe,” Duda told reporters.
He added: “New Russian units are arriving all the time, both at the Russian-Ukrainian border and in Belarus.”
The Polish head of state also said that “everybody is watching with concern” and wondering whether Russian units will “return to their home garrisons after the manoeuvres or stay in Belarus.”
Under this second scenario, Duda said, “we can say that Russia will gain an additional military district, and the military border between NATO and Russia will expand by 1,200 km, because such is the length of Belarus’ frontier with NATO.”
The president warned: “This is a real concern, we can’t ignore it, we must be mindful of it.”
Meanwhile, Stoltenberg stated that “NATO will do whatever is necessary to protect and defend all allies,” while maintaining its “open door” policy towards countries that wish to join the alliance.
“NATO can count on Poland, and Poland can always count on NATO,” he emphasised.
Planes carrying US troops and army equipment landed in Poland over the weekend as part of efforts to bolster NATO's eastern flank and reinforce allies in Eastern Europe amid a Russian military buildup near Ukraine.
Meetings with senior EU officials
Earlier in the day, Duda met with the president of the European Commission, Ursula von der Leyen, in Brussels.
His foreign-policy advisor Jakub Kumoch wrote on Twitter that the meeting focused on "the situation around Ukraine’s borders and the European Union’s response to the Kremlin’s aggression."
Kumoch added that Duda and von der Leyen had also spoken about Poland's relations with the European Commission and "Western unity in the face of shared threats."
The Polish president then held talks with the president of the European Council, Charles Michel.
After those meetings in the Belgian capital, Duda told reporters: “What we need today is European unity, both in the EU and NATO.”
He added: “Only through this route are we able to protect peace and international law; when international law is complied with, there is no threat of war.”
On Tuesday, the Polish president will discuss the situation around Ukraine at a meeting in Berlin with German Chancellor Olaf Scholz and French President Emmanuel Macron as part of the Weimar Triangle group, officials told reporters.
The Weimar Triangle was set up by Poland, Germany and France in the early 1990s.
Russia's military buildup near Ukraine has in recent weeks raised fears in the West that Moscow may be preparing for a new invasion of the country.
Moscow has denied plans for an assault but says it could take unspecified military action if its security demands are not met, the Reuters news agency has reported.
Russia annexed the Crimea peninsula from Ukraine in 2014 and then fomented a separatist conflict in that country's eastern Donbas region, leading to a wave of EU and US sanctions against Moscow and Russian officials.
(pm/gs)
Source: PAP