Polish Border Guard helicopter, patrolling the Poland-Ukraine border

The European Union has lifted border controls for citizens of nine of the newest EU member states as those countries join the union's vast passport-free zone.

The entry of the nine countries into the so-called Schengen zone will let EU citizens travel by land or sea through 24 European countries without facing border checks. Airports are scheduled to lift passport controls next April.

The expanded zone adds 3.6 million square kilometers to the visa-free area and includes more than 400 million inhabitants.

Travelers from outside the union will be able to move freely within the zone on a visa obtained from any of the Schengen member countries.

The nine new Schengen entrants are the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia.

Portugal, Spain, France, Italy, Belgium, Luxembourg, the Netherlands, Germany, Austria, Sweden, Finland, Denmark, Greece and non-EU members Norway and Iceland are already members of the zone.

Some information for this report was provided by AFP and Reuters.