Street in Hanoi, Vietnam Christine Zanino/ Wikimedia Commons HANOI (Reuters) - Vietnam will hand out fines of 100 million dong ($4,740) to anyone criticizing the government on social media, under a new law announced this week, the latest measure in a widening crackdown on dissent by the country's communist rulers.

Comments that did not constitute criminal offences would trigger fines if held to be "propaganda against the state", or spreading "reactionary ideology", according to the law signed by Prime Minister Nguyen Tan Dung.

Vietnam has repeatedly drawn fire for the harsh treatment and lengthy jail terms it has given to bloggers who criticized its one-party regime. The number of arrests and convictions has soared in the last four years.

The new decree is vaguely worded and did not say what comments amounted to a criminal offence, which can be punished with prison, or an "administrative violation" that rates a fine.

Internet penetration is soaring in a country of an estimated 90 million people, a third of whom use the internet and about 20 million of whom have Facebook accounts, a report published at a seminar on information technology in Ho Chi Minh City in September showed.

A Vietnamese Facebook user who campaigned online for the release of his brother jailed for criticizing the government fell afoul of the same law and was last month sentenced to 15 months of house arrest.

Rights groups and foreign governments have come down hard on Vietnam over its draconian cyber laws, including the United States, which has urged Vietnam to improve its human rights record to strengthen its case for stronger trade ties.

Media freedom campaigners Reporters Without Borders calls the country "an enemy of the internet".

The law would anger social media users, said Nguyen Lan Thang, a well-known Vietnamese Internet activist, who questioned the need for it.

"How could the government be destroyed by comments and the sharing of information on personal social media?" Thang said.

The decree also said anyone posting online a map of Vietnam inconsistent with the country's sovereignty claims faced fines.

The issue is hugely sensitive in Vietnam, where China's perceived encroachment of territory generates the kind of quiet anger experts say Vietnam's government wants to rein in.