The decision comes after some details of the secret proceedings under way ahead of the election were purportedly revealed to Italian newspapers.
The Vatican denied putting any pressure on the American cardinals to keep quiet. However, Vatican spokesman the Rev. Federico Lombardi said the Holy See considered this week's pre-conclave meetings, in which cardinals are discussing the problems of the church, to be secret and part of a solemn process to choose a pope.
"The College (of Cardinals) as a whole has decided to maintain a line of an increasing degree of reserve," Lombardi said.
Sister Mary Ann Walsh, the spokeswoman for the American cardinals, said Wednesday's briefing was canceled after other cardinals expressed concern in the morning "about leaks of confidential proceedings reported in Italian newspapers."
Walsh said all interviews had been canceled as a precaution. The Vatican has asked the cardinals to stop talking to the media.
She told the Associated Press that Italy's La Stampa newspaper had on Monday and Tuesday reported details of comments individual cardinals made in the closed-door meetings that were cited as a violation of their oath of secrecy. That prompted the decision to observe a media blackout.
She dismissed speculation that the Vatican and cardinals from other countries simply didn't appreciate the openness of the Americans, saying, "I don't think anyone was angry at the Americans. They were angry at La Stampa." She added that the Americans had been assured that the Vatican was pleased with their briefings.
However, Italian media speculated that Vatican-based cardinals were displeased with the popular American briefings and the U.S. openness about wanting to delay the start date of the conclave.
At Tuesday's briefing, Cardinals Daniel DiNardo of Texas and Sean O'Malley of Boston said they liked taking their time for pre-conclave discussions so they can gather information about possible problems of the Vatican bureaucracy and discern who among them should be pope.
"We need to give it the time that's necessary," O'Malley told the packed press conference at the North American College, the U.S. seminary up the hill from the Vatican. "I believe the feeling of the cardinals is that we want to have enough time in the general congregations so that when we go to the conclave itself it's a time of decision.
On the flip side, Italian newspapers reported Wednesday that Vatican-based cardinals wanted the election to take place quickly and that there was no reason to draw out the pre-conclave discussions. The implication was that perhaps they don't want all the Vatican's dirty laundry aired out.
Italian newspapers and international media have reported on the unique American briefings - and how they contrasted with the near-silence from other cardinals.
During Tuesday's briefing, DiNardo and O'Malley held a lively and informative 30-minute chat with some 100 reporters and two dozen television crews from around the globe. They revealed no details of their closed-door discussions. But they nevertheless provided journalists with insight about the process from two people actually involved.
"We're trying to help people have a greater understanding of what the process is and the procedures and background information," O'Malley told reporters. "Right now that's about all we can share with you but we're happy to try to do it."
Although the Americans were the only cardinals who were holding daily briefings, other individual cardinals have given occasional interviews to individual media.
And in an indication that the blackout wasn't total, U.S. Cardinal Timothy Dolan went ahead with his live radio show broadcast Wednesday.
"We're dealing with a journey, and part of that journey are the pre-conclave meetings that are taking place right now and then the conclave itself," said Father Thomas Rosica, a Vatican spokesman. "There is a certain respect that grows amongst the participants in the conclave as they get deeper into the conversations."
The Associated Press contributed to this report.
