New rules governing travel to and from China have come into effect, a move that observers say will further tighten the government's control over its citizens. Citizens deemed to have "harmed" China's national security or interests while overseas can also be barred from leaving China for up to three years after they return, while those who violate technological import and export regulations may also be banned from leaving. Chinese authorities say the rules are to "safeguard national sovereignty and security" and deny that they restrict the travel of "ordinary citizens". But critics say the new rules could further boost Beijing's ability to exert pressure on its nationals' activities abroad.