Sri Lankan police Friday shut down opposition news websites, accusing them of carrying "false and vulgar reports", months after imposing censorship on news alerts issued by mobile phone companies.

Seven media organisations, including the Sri Lanka Working Journalists Association, condemned the closure of the Lanka Mirror and Lanka X News offices located within the same building in Colombo.

"A group of 25 officers from the police Criminal Investigation Department sealed the offices and nine employees were arrested," the media organisations said in a joint statement. "We condemn this deadly assault on media freedom."

The Sri Lankan defence ministry said the news organisations were shut down after officials obtained a court order to carry out searches and seize equipment.

Rights groups and employees said one website was the official news organ of the opposition United National Party (UNP), while the other website was also closely linked to the UNP.

"The CID sealed the offices of two websites carrying false and vulgar reports," the ministry said in a statement. "People responsible for feeding information to the Internet through this office are being investigated."

The sites have been highly critical of the government, which is already facing international censure over its human rights record and a string of unsolved murders of journalists and attacks on the independent media.

Some 17 journalists and media employees have been killed in Sri Lanka in the past decade, pro-opposition websites have been blocked and media organisations opposing government views have been attacked, according to rights groups.

The latest police crackdown comes three months after the defence ministry ordered that all mobile phone operators should clear any security-related news reports before issuing them as SMS alerts.

Sri Lanka lifted a state of emergency last year, but media rights groups have said that journalists have been forced to self-censor their work amid fears of physical attacks.