Poland introduced temporary controls on its borders with Germany and Lithuania on Monday in an effort to stem what the government says is an increasing number of undocumented migrants crossing from the north and west, EDnews reports, citing Reuters.

The re-imposition of border checks is just the latest example of how mounting public concerns across the European Union over migration are straining the fabric of the bloc's passport-free Schengen zone. The Netherlands, Belgium and Germany itself have already implemented similar measures.

In Poland, the debate over migration has become increasingly heated in recent weeks, with groups of far-right activists launching "citizens' patrols" on the western border amid Polish media reports of German authorities sending undocumented migrants back across the frontier.

"Everything is proceeding without incident," Interior Minister Tomasz Siemoniak told private broadcaster TVN24 on Monday after the controls came into effect.

"Traffic is currently moving smoothly, 800 police officers, 200 gendarmerie soldiers, 500 territorial army soldiers, all services... are in full readiness."

Speaking at a midnight press conference on the German border when the checks began, Siemoniak also said only state officials such as border guards were authorized to check vehicles entering Poland, in a reference to the "citizens' patrols".

The border guard said in a post on X it had detained an Estonian citizen on Poland's border with Lithuania for transporting four illegal migrants believed to be Afghans.