WARSAW, Poland — The United States is preparing to boost its military presence in Europe at a cost of up to $1 billion, President Obama said Tuesday, as tensions in the region simmer over Russia’s aggressive actions in Ukraine.

Standing with Polish President Bronislaw Komorowski, Obama said the U.S. plans to send more military equipment and rotate additional American troops into the region. He called on lawmakers in Washington to provide the funding to sustain the effort.

Obama’s announcement marks a significant departure from a two-decade trend toward a smaller U.S. military presence in Europe, amid a shift by the Obama administration to a more visible and active naval and air power presence in the Asia-Pacific region. Just three years ago, the Pentagon downgraded the top U.S. Army Europe commander from a four-star to a three-star general.

“Today, I’m announcing a new initiative to bolster the support of our NATO allies here in Europe,” Obama said at Warsaw’s Belweder Palace. “Under this effort, and with the support of Congress, the United States will preposition more equipment in Europe.”

He called the proposal “a powerful demonstration of America’s unshakeable commitment to our NATO allies.”

If approved, the funding will be used to increase military exercises and training missions on the continent, as well as rotations of air and ground forces, the White House said. Officials said Obama also was seeking to ramp up U.S. Navy participation in NATO deployments in the Black and Baltic seas, plus working to boost the military capacity of non-NATO countries in Russia’s neighborhood, including Ukraine, Georgia and Moldova.

Obama’s announcement came at the start of a three-country swing through Europe steeped in both historical significance and regional anxiety over the crisis in Ukraine.

A day before his first face-to-face encounter with Ukraine’s newly elected president, Petro Poroshenko, Obama said he wants the U.S. and Ukraine to have good relations with Russia. But in a warning to Moscow, Obama said the U.S. has contingency plans to protect every member of NATO, and has been steadily developing those plans in recent years.

“Our contingency plans are not just pieces of paper on a shelf,” Obama said, adding that the U.S. must and does have the ability to put those plans into effect if needed.

The cautionary notes from Obama and Komorowski came a few days before a potential encounter between Obama and Russian President Vladimir Putin, who also planned to be in France for the 70th anniversary of the D-Day invasion that eventually led to Allied victory in World War II.

Obama and Putin haven’t met in person since the Ukraine crisis began and have no meetings scheduled, but White House officials haven’t ruled out that they could cross paths.