Iran has arrested 14 journalists for alleged co-operation with foreign-based Persian-language media organisations.

Several chief editors of Iranian outlets said on Monday that the arrests signal a major escalation in a press crackdown that reflects Iran's zero tolerance for those who work with dissident media or outlets considered hostile to the regime.

The chief editors of the arrested journalists told The Associated Press that the 14 were taken into custody on Sunday night and on Monday because of their "foreign contacts".

The editors spoke on condition of anonymity because of fears of reprisals.

In recent years, Iran has denounced the Voice of America (VOA) and the BBC's Persian service, describing them as arms of US and British intelligence agencies. It has warned of severe repercussions for Iranian journalists and activists caught having contacts with these outlets.

The editors refused to say if the detained were accused of providing material specifically to BBC or the VOA .

Tehran has repeatedly accused the United States and Britain of seeking to provoke unrest in a bid to oust the country's clerical rulers, and it has frequently accused opposition figures and supporters of being in league with the nation's enemies.

Pre-election tensions

Editorial cartoonist Mana Neyestani, speaking to Farsi-language Raha TV, a UK-based opposition channel, said that the journalists arrested were not known for being particularly political, but they were all fairly well known.

"It seems that they wanted to go for prominent, well-known names and take them in a dramatic style to make a point," said Neyestani, adding that these arrests are part of a pre-emptive crackdown on journalists in the lead up to the June presidential elections.

"I can't even say if I'm shocked, because for quite some time we've forgotten the point of being shocked with our country's system - no news really shocks us any more," said Neyestani, himself in exile in France.

Sherif Mansour, the Middle East and North Africa Program Co-ordinator for the New York-based Committee to Protect Journalists, told Al Jazeera that Iran had been ranked as "the worst jailer of journalists worldwide".

"And since the 2009 election, Iran has kept, and continued pressure, jail-time, torture and even lashing sentences against journalists," said Mansour, referring to the contested presidential re-election of President Mahmoud Ahmadinejad, which lead to months of protests and massive and persistant crackdowns on media as well as political opposition.

"We see the escalation coming as the election time comes," said Mansour.

"We're also seeing more consistent use of oppression tools by increasing censorship online, cracking down on some of the foreign journalists, Iranian journalists and human rights activists."

'Serving the enemy's purpose'

The arrests followed last week's warning by State Prosecutor Gholam Hossein Mohseni Ejehei, who said journalists who are in contact with "hostile foreign media" would be punished.

Ejehi described such reporters as "serving the enemy's purpose".

The detained journalists - nine men and five women - were identified by their editors.

They are from seven different news organisations, including five daily papers, a weekly and the semi-official ILNA news agency.

Some of them have spent months behind bars in the past over critical writings that were carried by foreign-based Iranian dissident media or writings that supported feminist activities.

Since 2000, Iran's judiciary has shut down more than 120 pro-reform newspapers and jailed dozens of editors and writers on vague charges of insulting authorities.

Also on Monday, the semiofficial ISNA news agency said the conservative Tabnak news website, close to the former Revolutionary Guard commander Mohsen Rezaei, has been blocked. No reason was given.