More than 20,000 foreign media personnel are due in the Chinese capital to cover the Olympic Games, which begin on 8 August.
Many are already moving into the press and broadcast centres in Beijing.
On Tuesday, they were unable to access the website of Amnesty International as it released a report criticising China's human rights record.
Some international news pages and sites that dealt with issues such as Tibet were also inaccessible, journalists said.
Chinese foreign ministry spokesman Liu Jianchao confirmed that websites relating to the Falun Gong spiritual movement were blocked.
"As to sites related to Falun Gong, I think you know that Falun Gong is a cult that has been banned according to law, and we will adhere to our position," he told a news conference on Tuesday.
He suggested that part of the problem with other sites could lie with the sites themselves.
The press will be accommodated in a high-tech facility in Beijing
"There are some problems with a lot of websites themselves that makes it not easy to view them in China," he said. "Our attitude is to ensure that foreign journalists have regular access to information in China during the Olympic Games."
But on Wednesday, a spokesman for the Olympic organising committee told the French news agency AFP that other - unspecified - sites were blocked.
Sun Weide declined to provide more details when asked by the BBC.
But he said reporters would be able to do their jobs.
"During the Olympic Games we will provide reporters with sufficient and convenient internet access so the Olympic Games will not be affected," he said.
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