WASHINGTON (CNN) - Thirteen journalists were killed worldwide in 1998 and nearly 100 others remain in prison in 25 countries, spokesmen for Reporters Sans Frontieres (Reporters Without Borders), said Sunday.

The most dangerous countries for journalists were Mexico and Colombia, the group said. Just last month, two journalists were assassinated in Mexico.

The reporters' group denounced the attacks on journalists and said it would continue to seek release of imprisoned journalists. Many journalists have been attacked because they refused to submit to censorship or reported embarrassing facts.

In freedom of press, Africa also lags, RSF said. Among countries where press freedoms were repeatedly violated, the report named Eritrea, Tunisia, Cameroon, Angola and Ethiopia.

Reuters contributed to this report.