The US and Cuba do not have full diplomatic relations, and Washington has maintained an economic embargo on the only one-party communist state in the Americas, for decades.
The visiting US politicians have been pressing for the US government to lift the embargo and resume a dialogue with Havana.
Fidel Castro has not been seen in public since July 26, the day before his surgery, although he has appeared in videotaped visits in his hospital room.
The timing for the unprecedented visit came hours after, John Negroponte, the US director of national intelligence, said that Fidel Castro was close to death.
Negroponte said: "Everything we see indicates it will not be much longer ... months, not years."
The US politicians' arrival in Cuba came on the day that Hugo Chavez, the Venezuelan president, denied reports that Fidel Castro has cancer at all, saying that he was instead fighting a "great battle" against a "serious" illness.
Fidel, 80, a close ally of Chavez, temporarily stepped aside on July 31 to his brother and defence minister Raul, 75, after maintaining more than four decades of fierce resistance to Washington.
Raul Castro has reached out to Washington more actively than his brother in the four months of his temporary leadership. On December 2 at a military parade that Fidel sat out, Raul called for negotiations between the two countries.
No liberalisation, no talks
However, the US has indicated no interest in talks without signs of political liberalisation in Cuba.
On Wednesday, Tom Shannon, the most senior US diplomat for Latin America, said that Washington had yet to see any reformers in the Cuban government.
He said: "Once [Fidel Castro] goes, the successor government is going to have to chart out some kind of path into the future. There are no clear signals of what that path is going to be."
"We don't see any significant possibility of change of any kind until Fidel is gone."
Shannon said that any US engagement with Cuba must be "part of a change process that facilitates a democratic transition".
"We are attentive to what is happening in Cuba, to what would happen after Fidel Castro passes from the scene. When we engage it has to be part of a democratic change".