The announcement gave no details about which groups might be targeted but Beijing sometimes accuses political, human rights, Tibet and other activists abroad of trying to undermine communist rule.
The move is part of efforts to step up a crackdown on online smut, gambling, fraud and other offenses, said Wang Chen, chief of the Cabinet's Information Office, according to the official Xinhua News Agency.
"We will strengthen the blocking of harmful information from outside China to prevent harmful information from being disseminated in China and withstand online penetration by overseas hostile forces," Wang was quoted as saying.
Beijing encourages Internet use for business and education but tries to block material deemed subversive or pornographic and operates an extensive system of Web monitoring and censorship. Regulators block access to websites abroad run by political and human rights activists and some news organizations.
U.S.-based Google Inc. moved its China-based search engine to Hong Kong last month after announcing it no longer wanted to cooperate with Chinese online censorship.
China's population of Internet users, already by far the world's largest, has surpassed 400 million, with 233 million users getting access through mobile phones, according to weekend reports by state media, also citing Wang.
Beijing has launched repeated crackdowns against online pornography and fraud and has taken steps in recent months to block use of the Web by activists to spread criticism of the government.
China on Thursday strengthened a law to require telecommunications and internet companies to inform on customers who discuss state secrets - an area so broadly defined that both companies and Chinese citizens have struggled to know just what a state secret is.
In February, the government announced that individuals who want to operate websites must meet in person with regulators and submit photos of themselves.
In April, three people who posted material online about the death of a pregnant woman in police custody were sentenced to prison by a court in southern China. They were convicted of slander.