In a keynote address in Warsaw, U.S. President Donald Trump urged Russia “to cease its destabilizing activities in Ukraine and elsewhere" and specifically reaffirmed Washington's commitment to NATO's Article 5.
Trump spoke on July 6 before a cheering, flag-waving crowd of 10,000 people in historic Krasinski Square in central Warsaw, where former President Lech Walesa sat among guests in the VIP area.
During the nationally televised speech, Trump also said Moscow should end its "support for hostile regimes -- including Syria and Iran.”
He called on America's NATO allies to join the United States in a defense of the West against "radical Islamic terrorism" and "any form of ideological support" that terrorists receive.
Trump said the United States repeatedly "has demonstrated not only with words, but with its actions, that it stands behind Article 5" of the NATO treaty -- the provision stating that an attack on one NATO country is an attack on all members of the alliance.
Many observers were closely watching Trump's comments about the treaty provision after he shocked NATO allies during a May meeting in Brussels by failing to mention it.
Returning to one of his oft-repeated demands, he said all NATO members must meet their financial commitments to the alliance, and he praised Poland for being one of the few NATO countries that has done so.
Trump described Poland as a long-time U.S. ally that is "an example for others who seek freedom and who wish to summon the courage and the will to defend our civilization."
Earlier on July 6, after meeting with Polish President Andrzej Duda, Trump said the United States is "committed to maintaining peace and security in Central Europe" and again said that Washington is "working with Poland in response to Russia's actions and destabilizing behavior."
Any comments by Trump on Russia are closely watched in Poland.
The country's proximity to its rival has heightened its concerns about Moscow’s intentions in the region, especially after Russia’s illegal annexation of Ukraine’s Crimea region, its support of separatists in eastern Ukraine, and its alleged efforts to manipulate elections in the United States and some European Union countries.
The White House gave its blessing to a $7.6 billion deal to sell Patriot missile defense systems to Poland by the end of the year. U.S. missile defense systems in Eastern Europe have prompted repeated complaints from Russia in the past, and President Barack Obama had shelved such plans in 2009, as part of his administration’s attempted “reset” of relations with Moscow.
A Kremlin spokesman said Moscow disagrees with Trump’s description of Russia’s behavior as “destabilizing” and said the Kremlin regrets that there is a lack of understanding between Russia and the United States.
Trump told reporters after talks with Duda that he thinks Russia meddled in the 2016 U.S. presidential election, but said he also thinks "other people and, or, countries" may have interfered in the election campaign.
"Nobody really knows for sure," Trump said.
In Warsaw, Trump told a summit of Central and Eastern European leaders that "America will be your strongest ally and your steadfast partner" in their attempts to reduce the region's dependence on Russian energy supplies.
Trump encouraged leaders at the Three Seas Initiative summit to take advantage of newly available supplies of U.S. liquefied natural gas.
The Three Seas Initiative is a joint project of Poland and Croatia that was launched in 2016 with the aim of strengthening trade, infrastructure, and cooperation on energy and politics between countries that border the Adriatic Sea, the Baltic Sea, and the Black Sea.
WATCH: Trump Affirms Commitment To NATO, Security In Europe
Other countries involved in the initiative include Hungary, the Czech Republic, Slovakia, Romania, Bulgaria, Lithuania, Estonia, Latvia, Slovenia, and Austria.
After Warsaw, Trump traveled to Hamburg, Germany, for the Group of 20 summit, where he plans to meet on July 7 with Russian President Vladimir Putin.
He held talks with German Chancellor Angela Merkel shortly after arriving. The two leaders shook hands and spoke briefly before leaving for closed-door talks.
Trump also will hold one-on-one talks with British Prime Minister Theresa May in Hamburg on July 7, May's office announced.
He is also expected to attend a Northeast Asia Security dinner with South Korean President Moon Jae-in and Japanese Prime Minister Shinzo Abe.
North Korea's intercontinental ballistic-missile test earlier this week was expected to be high on that agenda.
A demonstration, which organizers have labeled G20: Welcome to Hell, was planned for the night of July 6.
WATCH: Trump To Promote U.S. Gas During Warsaw Visit
Some 20,000 police have been deployed in Hamburg to provide security during the G20 summit.
Hamburg police clashed with demonstrators on the night of July 5 ahead of the meetings, using water cannons to disperse some protesters.
Germany has said that among tens of thousands of protesters expected to converge on Hamburg during the summit, as many as 8,000 could potentially be involved in rioting and other violence.
With reporting by AP, Reuters, dpa, CNN, CNBC, and The Wall Street Journal