The U.S. Secretary of the Treasury Janet Yellen met with Polish Prime Minister Mateusz Morawiecki and other officials on Monday to discuss Poland's opposition to a global 15 percent corporate minimum tax and the Ukraine crisis.

The visit to Warsaw is the first in a series of meetings for Yellen in Europe. She will also travel to Brussels in Belgium and Bonn in Germany this week.

Poland is the only country in the European Union (EU) that has so far resisted plans to put a minimum on corporate taxes, aimed at halting a "race to the bottom" of countries trying to attract foreign enterprises.

Poland's main argument against the tax is that it would put a larger burden on European companies, unless it is linked to changes in tax treaties that would allow countries to levy more taxes on large U.S. technology giants. This so-called Pillar 1 plan has however not been fully developed yet.

The U.S. Treasury urged Poland to move on with the tax deal, saying it would "raise crucial revenues to benefit the citizens of both Poland and the United States."

"We've had very good discussions, good, frank discussions, in all of the meetings, and we'll continue to work on all of the issues," Yellen said.

The two sides also discussed the Ukraine crisis, including sanctions on Russia. Yellen expressed gratitude to Poland for admitting 3.4 million Ukrainian refugees since the start of the crisis.