Obama in Poland: Europe security is 'sacrosanct'

US President Obama has arrived in Poland to mark the 25th anniversary of the collapse of communism, in a visit expected to be dominated by the crisis in eastern Ukraine.

The president said America's commitment to the security of Poland and Central and Eastern Europe was "a cornerstone" of its own security, and was "sacrosanct".

He added that the US had boosted its support for Poland because of events in Ukraine.