Two Australian journalists fled China Tuesday under diplomatic protection, fearing arrest as political pawns in the rapidly worsening relationship between Canberra and Beijing.

Their dramatic overnight exit came following days of secret wrangling that had seen both men holed up in Australia's diplomatic missions to escape the clutches of China's feared security police.

Bill Birtles and Michael Smith had to consent to question before they were allowed to leave China, shepherded out of the country on a late-night flight by Australian diplomats.

Both men were quizzed about fellow Australian journalist Cheng Lei, who has been detained since last month.

Beijing acknowledged Tuesday for the first time that she was being held on national security grounds -- a broad category that can include crimes resulting in lengthy prison sentences.

Birtles, who works for public broadcaster ABC, said Tuesday his "interrogation" in a hotel room had touched on Cheng's case, but that he did not think that was its primary objective. "I believe that the whole episode was really one more of harassment of the remaining Australian journalists, rather than a genuine effort to try and get anything useful for that case," he told ABC.

China confirmed the two men had been questioned, but insisted the move had been legitimate. "As long as foreign journalists obey the law... they have no reason to worry," foreign ministry spokesman Zhao Lijian said in Beijing.

Several reporters for US media have had their visas revoked and been forced to leave the country -- what critics have interpreted as targeting of Western media outlets by the Communist Party.

The Foreign Correspondents' Club of China said the two men's ordeal marked "a significant escalation of an ongoing, sustained Chinese government assault on media freedoms".

"Such actions by the Chinese government amount to appalling intimidatory tactics that threaten and seek to curtail the work of foreign journalists based in China, who now face the threat of arbitrary detention for simply doing their work."

The episode began almost a week ago, when police carried out synchronised midnight raids on Birtles's home in Beijing and Smith's in Shanghai -- where he was correspondent for the Australian Financial Review.

Both were barred from leaving the country and told they would face questioning. "I felt like I suddenly, unintentionally, had become a pawn in some sort of diplomatic tussle," said Birtles.

Fearful of interrogation and arbitrary arrest, they fled to their closest Australian diplomatic missions until agreeing to answer Ministry of State Security questions in return for safe passage home.

Crackdown 

James Curran, an Australian former intelligence analyst and prime ministerial adviser, told AFP the media crackdown and the fact no major Australian media were now represented in China would only hinder public understanding.

"The lens through which we are going to get a picture of this country is going to become increasingly shrouded, if not blocked altogether," he said.

Author and former China correspondent Richard McGregor said the incident "marks a new low". "Other countries grappling with China will take note. If their bilateral relationship deteriorates, then their own nationals will be in the firing line as well."

Australia has increasingly pushed back against what it sees as China's more aggressive projection of power and influence across Asia since President Xi Jinping took power in 2013.

Curran said the problems stem from Xi's pursuit "a very aggressive form of Chinese exceptionalism", with nationalism increasingly being used as "the glue holding the country together". 

"They just seem to be taking the view that as a rising power they should be able to flex their strategic, ideological and rhetorical muscle, and that other countries are just going to have to put up with it."

Beijing was particularly infuriated by Australia's role in international calls for an investigation into the origins of the coronavirus pandemic, which emerged in the Chinese city of Wuhan.

A Chinese envoy in Canberra recently portrayed Australia's behaviour as a betrayal that caused "indignation, anger and frustration" in China. Since then, China has taken steps to curb key Australian imports and encouraged Chinese students and tourists to avoid the country.