The powerful head of Poland's ruling party wants his country to be "included" in the US nuclear defence system, according to an interview published Wednesday.

Jaroslaw Kaczynski told the Gazeta Polska daily that Warsaw "should work for the inclusion of Poland in the American nuclear defence system", which "would be the optimal solution".

But Kaczynski also called for Europe to become a "nuclear superpower", in a separate interview published Tuesday by Germany's Frankfurter Allgemeine Zeitung daily, as Chancellor Angela Merkel visited Warsaw.

Under NATO's Article 5 collective defence agreement, the US nuclear umbrella already protects NATO allies including Poland as well as Japan, South Korea and Australia.

Kaczynski, who is regarded as Poland's most powerful decision-maker despite holding no formal governmental post, did not elaborate how his proposed arrangement would be different from the existing Article 5 security guarantee.

His comments come amid uncertainty in eastern NATO allies like Poland stoked by US President Donald Trump's seemingly pro-Moscow stance coupled with critical pronouncements on NATO.

Trump has called the alliance "obsolete" in terms of fighting terrorism, but of "fundamental importance" to transatlantic security.

Kaczynski told Germany's FAZ he "would welcome" a European nuclear umbrella.

"Europe would become a superpower. One or two nuclear submarines would not be enough," Kaczynski told the FAZ, referring to Britain's ageing fleet of nuclear-armed submarines.

Greatly reduced after the end of the Cold War, the US nuclear arsenal in Europe includes around 200 tactical weapons stationed in Germany, Belgium, Italy, the Netherlands and Turkey.