Polish police and border guards have stepped up security checks at airports and border crossings as the search intensified for the infamous sign stolen from the Auschwitz death camp memorial.

The 16ft sign – which read "arbeit macht frei" (work makes you free) – was stolen early on Friday.

Interior minister Jerzy Miller ordered police to question all possible witnesses and suspects in a nationwide effort to find the sign.

The director of the Auschwitz-Birkenau memorial museum said he believed it was carried out by professionals and that none of the memorial museum's staff were considered suspects.