BEIJING (Reuters) - Delegates at China's annual Parliament session on Wednesday were loath to discuss Mr Zhou Yongkang, the former domestic security chief rumoured to be the most senior politician ensnared in a graft scandal in modern China's history.

Speculation has swirled for months that Mr Zhou, 71, who was a member of the ruling Communist Party's Politburo Standing Committee, the apex of power, is being investigated for corruption.

The party, though, has made no announcement on Mr Zhou, who held the immensely powerful post of security overlord until he retired in 2012.

Mr Zhou has been put under virtual house arrest while the party investigates accusations of corruption against him, sources have told Reuters. The case would be highly sensitive for the party, which always wants to project an image of unity, and at the same time it would be a big test of how far it will go in the fight against graft.

Mr Zhou is just the type of powerful figure, or "tiger", that President Xi Jinping has vowed to net in his corruption campaign.

One delegate at the opening of the National People's Congress in Beijing's Great Hall of the People, was prepared to offer a few words on the case.

"Zhou Yongkang is a national leader, if he made a mistake, he must be investigated, if he broke the law, we should use the legal system to control it," Mr Wang Jiaqi, a delegate from north-eastern Jilin province, told Reuters.

"But we can't frame a good person, right? We must respect the law and have the facts," added Mr Wang, who said he had heard about speculation surrounding Mr Zhou from colleagues at work.

Premier Li Keqiang, in a wide-ranging work report to parliament, repeated a pledge to fight corruption, saying the government would "penalise offenders without mercy", though he unveiled no new plans to tackle the problem.

Most lawmakers declined to discuss what they thought of Mr Zhou, with some saying they had not even heard of reports he was being investigated.

"We shouldn't be talking irresponsibly about hearsay," said Ms Cui Liru, a delegate to the mostly ceremonial advisory congress that meets in tandem with Parliament.

Others quickly moved away, with several objecting to a Reuters reporter about asking such a question.

"I cannot say, I really don't know, don't ask me," said Mr Qi Zhenwei, a delegate from central Hunan province.

Mr Zhou, along with other retired party officials, had not been expected to put in an appearance at the session. He has not been seen in public since October, when he attended an alumni celebration at the China University of Petroleum.

"The central government should have a decision on his matter," said Mr Xu Juyun, a delegate from Hunan. "I'm just an ordinary delegate, I don't know the circumstances of these sensitive matters."

In a rare hint in state media, the Global Times newspaper said this week in a commentary: "It seems that the investigation into Zhou hasn't concluded yet."

The spokesman for the largely ceremonial advisory body to parliament said on Sunday, when asked about Mr Zhou, the government was committed to fighting corruption, no matter how senior any suspect might be.

He then concluded by saying: "I'm sure you understand."

That quickly became a catchphrase that spread across China's Internet, and has been interpreted as code that Mr Zhou is, indeed, in trouble.

Asked for his assessment on Mr Zhou, Mr He Junming, a delegate from Sichuan province, paused for a few seconds then said: "On this, I should say, you understand too."

He then walked away from a group of amused reporters.