Secretary of Energy Rick Perry met with Polish and Ukrainian leaders on Saturday in Warsaw, where he said that it is likely that the United States will toughen sanctions on Russia due to the 2014 annexation of Crimea.

Polish President Andrzej Duda said at the conference that Western sanctions need to continue over the Crimean annexation.

Poland has become increasingly nervous about Russian activity in Eastern Europe and has in the past tried to get President Trump to build a military base in Poland. The name could be called "Fort Trump," as a way to persuade the president and appeal to his ego.

Ukrainian President Volodymyr Zelensky said at the meeting that he had been working with Poland to try to come to a resolution on the eastern Ukraine conflict and Crimea's annexation. "We have agreed on our next steps to stop the war in eastern Ukraine and to bring back occupied Crimea," Zelensky said.

A major issue earlier this month was whether to bring Russia back into the G7 countries, as its membership was suspended in 2014 due to the Crimea annexation. Zelensky opposed Russia's reentry, while President Trump has supported Russia's return.

Trump has also held up $250 million in military assistance to Ukraine, in order to verify whether the money is used in the best interest of the U.S.

The Trump administration has stated that it wants to make sure Ukraine is "paying its fair share."

Russia annexed Crimea between February and March 2014, to international condemnation. The strategically located peninsula in the Black Sea is a major asset for Russia, with the Kremlin justifying the move by saying the territory is mostly populated by ethnic Russians.

Trump has frequently blamed President Obama for the move, saying that "it was President Obama who allowed it to happen" and that Putin "outsmarted" Obama.

The Crimean annexation also set off a violent war in eastern Ukraine, with pro-Russian separatists fighting against Ukrainian forces. Human rights organization Freedom House has said that over 3,000 civilians have been killed in the conflict since 2014.