Prince Rainier III (search) of Monaco, who was hospitalized two weeks ago with a chest infection, has been moved to the intensive care unit, the palace announced Tuesday as his children rushed to his bedside.

After a marked earlier improvement, "a recurring pulmonary infection required that the prince be transferred into an intensive care unit," a palace statement said.

Rainier's three children were paying visits to their father on Tuesday, an official with knowledge of the situation told The Associated Press. Two of them had returned home from abroad to visit their father.

Rainier, 81, whose movie-star wife, Grace Kelly (search), died in a car crash in 1982, has a history of heart problems and has suffered poor health in recent years. He has been hospitalized numerous times over the past decade.

Rainier was admitted March 7 to the Monaco Cardio-Thoracic Center (search) due to what was described as a bronchial-pulmonary infection, a recurring problem.

The palace said on March 10 that the initial infection had been "quickly brought under control." Rainier's doctors then decided he should undergo a series of cardiac examinations, given his history of heart problems.

Doctors have not detailed the nature of Rainier's infection, but lung infections, such as pneumonia, are common — and often life-threatening — in older people.

"Someone in their 80s could develop multiple types of infections which are life threatening," said Dr. Michael Freedman, a gerontologist and professor at New York University School of Medicine. "Pneumonia puts a strain on the heart and often causes congestive heart failure."

Patients with chest infections are usually put into intensive care for two reasons: the infection is not under control and is causing breathing difficulties that require a breathing tube be threaded down the throat, or because doctors want to check for heart failure, Freedman said.

With news of his move into intensive care, Crown Prince Albert, 47, traveled from abroad to visit his father as did 48-year-old Princess Caroline, said an official familiar with the situation. The official asked not to be named. Princess Stephanie, 40, was seen entering and leaving the hospital.

Rainier had spent a week at the same Monaco hospital in October for treatment of a chest infection. He also was admitted to the clinic in February 2004 for a coronary lesion and a damaged blood vessel, and spent three weeks there the month before for what was described as general fatigue.

Rainier assumed the throne in 1949. His heir is Albert, who is unmarried and has no children. Monaco changed its succession law in 2002 to allow power to pass from a reigning prince who has no descendants to his siblings. Both Princess Caroline and Princess Stephanie have children.

Monaco, a tiny principality famous for its casinos, Formula One Grand Prix and tax breaks that attract the rich and famous, is nestled on the Mediterranean Coast between Italy and the French Riviera.