An intense security operation's in force in the US for the visit of Pope Francis.

He's just landed at Andrews air force base in Maryland.

It's the Pope's first visit to the United States.

At the end of the week he'll address 170 heads of government at the United Nations - a gathering John Key will attend.

CNN's Deborah Feyerick said planning's been going on for nine months.

"They're looking at everything - even who's up in the windows, who's up in the balconies, how do the doors open, who's in the bushes - its pretty extensive."

US Correspondent Richard Arnold said security for his trip is being treated as seriously as that of the Super Bowl, presidential inauguration and the Olympics.

"His visit will bring the largest security operation in US history, the visit has been designated as a special security event - a rare designation that is used for things like presidential inauguration, Super Bowl and the Olympics."

Francis is only the fourth pope in America's history to visit.