Rau held talks with his Ukrainian counterpart Dmytro Kuleba during the visit, which officials in Warsaw described as "extraordinary and urgent" and "associated with a threat to peace at Ukraine's borders."
The trip on Thursday came after a build-up of Russian forces near Ukraine's eastern border.
"An independent, secure Ukraine is an inalienable part of a Europe that remains true to the fundamental principles of the Charter of Paris for a New Europe, whose 30th anniversary we recently celebrated," Rau said afterwards, as cited by the Polish foreign ministry on its website.
He added: "In my talks with the Minister of Foreign Affairs of Ukraine, Dmytro Kuleba, Leonid Kravchuk, head of the Ukrainian delegation at the Trilateral Contact Group, and Oleksii Reznikov, Deputy Prime Minister for Reintegration of the Temporarily Occupied Territories of Ukraine, we discussed the threats to peace in Europe posed by the Nord Stream 2 gas pipeline, now in its final construction stages, and by the escalating security situation in Ukraine’s east and south."
Rau said the contested gas pipeline being built from Russia to Germany, if completed, "will create a state of clear and present danger to peace in Europe."
He added: "We agreed that the massing of troops along Ukraine’s eastern border and in the occupied territory of Crimea we are witnessing in recent days should be taken into account when shaping and conducting policy of non-recognition of the annexation of Crimea and violations of the territorial integrity of Ukraine.
"Depending on the course of events, Ukraine’s partners should be prepared to have a serious debate on the scope and criteria of selective cooperation with states that are in breach of fundamental norms of international law."
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