US President Barack Obama has proposed an initiative of up to $1 billion to finance extra US troop, air and naval deployments to "new allies" in eastern Europe amid a showdown over Ukraine.

The "European Reassurance Initiative," which must be approved by Congress, would also build the capacity of non-NATO states including Ukraine, Georgia and Moldova to work with the US and the Western alliance.

The effort would involve increased participation by the US Navy in NATO naval force deployments, including "more persistent deployments" to the Black and Baltic seas, it said.

"We are reviewing our force presence in Europe in light of the new security challenges on the continent," the White House said in a statement.

"These efforts will not come at the expense of other defence priorities, such as our commitment to the Asia Pacific rebalance." 

US President Barack Obama is in Poland for the first stop in a four-day visit to Europe.

The trip will culminate at a gathering of world leaders to commemorate the 70th anniversary of the Allied landing in Normandy.

Mr Obama is due to hold talks with Ukrainian president-elect Petro Poroshenko, but will not have a formal meeting with Russian President Vladimir Putin.

The US president travels to the Polish capital Warsaw today to take part in a celebration to mark the 25 years since Poland emerged from communism.

The White House has said Mr Obama will hold up Poland as an example for Ukraine, as well as restating America's "ironclad security commitment" to the defence of all NATO allies.

While in Poland, Mr Obama will also meet the new president of Ukraine - something of a stark contrast with the now ice-cold relationship between the US and Russia.

Tomorrow, Mr Obama will travel to Brussels for a meeting of the leaders of the world's seven most industrialised countries.

This gathering was supposed to take place in Russia, but was cancelled after Russia annexed Ukraine's Crimean Peninsula.

On Friday, Mr Putin will be among the world leaders who attend a ceremony at Sword Beach in Normandy to commemorate the Allied landings in World War II.

However, the White House has said Mr Obama will not have any formal meeting with Mr Putin.