President Barack Obama has called on Congress to approve up to 1 billion dollars to support eastern European countries, which he says "will be a powerful demonstration of America's unshakable commitment to our NATO allies".
On the first of a four-day European trip, Obama promised support in the face of Russia's actions in Ukraine, which includes "rotating additional ground troops and F-16 aircraft into Poland".
Poland and some of its NATO neighbours have been asking for extra troops for months. However, a big increase in US forces could prompt Moscow to take similar actions.
Speaking in a news conference in Warsaw he said that while maintaining sanctions against Russia, the US still wants good relations with the Kremlin.