President Obama has called on Congress to support a $1 billion plan to ramp up the U.S. military presence across Europe amid tensions between Russia and Ukraine. Obama made the announcement in Poland on the first leg of a four-day Europe trip. Shortly after his arrival in Warsaw, Obama said the United States has already boosted ground troops and F-16 aircraft in Poland.

President Obama: “I’m starting the visit here because our commitment to Poland’s security, as well as the security of our allies in central and eastern Europe, is a cornerstone of our own security, and it is sacrosanct. And during my visit here three years ago, I said that the United States would increase our commitment to Poland’s security. The United States honors our commitments, which you see in the aviation detachment at Lask Air Base. It is a commitment that is particularly important at this moment in time.”

Obama’s visit comes as Ukrainian forces have launched an offensive to retake the eastern city of Slovyansk from pro-Russian rebels. The assault follows deadly fighting Monday in the eastern city of Luhansk.