PHILADELPHIA (AFP/REUTERS) - Pope Francis, who has been limping more and more as his US trip goes on, has a hip ailment and gets physical therapy on a regular basis, the Vatican spokesman said Saturday.

The 78-year-old pontiff, currently on a whirlwind tour of Cuba and the United States that ends Sunday, has been observed walking slowly with faltering steps and getting help from accompanying priests when going up or down stairs.

Following an arduous schedule in New York and Washington on the back of his four-day trip to Cuba, Francis stumbled several times as he climbed the stairs of the American Airlines jet taking him from JFK to Philadelphia.

But he smiled and waved once he reached the plane door, apparently in good shape, and the plane took off.

"He has some problems in the movement of his legs. Some days are better than others," Vatican spokesman Federico Lombardi told reporters in Philadelphia when asked about the Pope's condition that has become more and more noticeable.

"During a trip like this, he gets a little tired. But he sleeps very well."

On a regular day at the Vatican, Francis usually wakes up at four or five in the morning to tackle a packed work schedule that involves meeting many people.

While on trips, the Pope's days are even busier. While he sometimes cancels appointments at the last minute, he has also been known to tack on extra agenda items.