NEW YORK -- At least 25 countries around the world block Web sites for political, social or other reasons as governments seek to assert authority over a network meant to be borderless, according to a study out Friday.

The actual number may be higher, but the OpenNet Initiative had the time and capabilities to study only 40 countries and the Palestinian territories. Even so, researchers said they found more censorship than they had initially expected, a sign that the Internet has matured to the point that governments are taking notice.

China, Iran, Myanmar, Syria, Tunisia and Vietnam had the most extensive filters for political sites. Iran, Oman, Saudi Arabia, Sudan, Tunisia, the United Arab Emirates and Yemen had the strictest social-filtering practices, blocking pornography, gambling and gay and lesbian sites.

In some countries, censorship was narrow. South Korea, for instance, tends to block only information about its neighboring rival, North Korea.