The president convened a meeting with cabinet ministers on Tuesday to discuss Russia's decision on Monday to recognise two breakaway regions in the east of Ukraine.
"Ukraine is constantly under immense threat and Russian troops have entered the breakaway regions which separatists have held for years," Duda said. "Now these separatists are being regularly reinforced by regular Russian units.
"The scenarios for the future may be different," the president went on to say.
Duda insisted that the EU and Nato should remain united.
"We believe that serious sanctions are needed," Duda said.
"The coming days will be marked by very intensive consultations in various formats, both within Nato and the EU," Duda continued. "We must act, as we strongly believe everything must be done to stop an invasion of Ukraine."
Among the topics for consultations, the Polish president mentioned a potential further strengthening of Nato's eastern flank.
"We must talk about our security architecture, among ourselves and with our allies," he added.
"Today the Russian army is in Belarus and there are numerous indications that it may not pull out," the president warned. "This requires us to take adequate steps in our defence policy and we also believe this requires action on the part of Nato." (PAP)