China's ruling Communist Party declared its General Secretary Xi Jinping the "core" of its leadership on Thursday, elevating his already powerful status.

A communique issued by top party leaders after a four-day meeting in Beijing called on all its members to "closely unite around the CPC Central Committee with Comrade Xi Jinping as the core", said the People's Daily, the party's official mouthpiece.

The announcement followed a gathering of 400 top party leaders in Beijing for a meeting known as the Sixth Plenum to discuss changes to party structure and discipline.

Scenes of the meeting shown on state broadcaster CCTV showed a relaxed but businesslike Xi, clad in a black windbreaker, lecturing a ballroom of rapt party members.

He has sought to bend the party to his will since taking its helm in 2012, and has already taken control of more levers of power than any leader since Mao Zedong.

Regional cadres began using the term "core" for Xi last December, but it then disappeared, suggesting that the Chinese president had encountered resistance to his efforts to further consolidate his power.

Analysts have speculated that Xi could seek to stay in power beyond the traditional 10-year term.

The declaration was "very significant", Willy Lam, professor of politics at the Chinese University of Hong Kong, told AFP, because in Chinese politics the "core" has traditionally denoted a degree of individual authority unconstrained by term limits.

"The core of leadership can last forever," he said. "There's no idea of tenure, retirement age associated with the core."

China has a constitutional limit of two five-year terms for the national president, another of Xi's titles, but no formal rule on tenure for the general secretary of the ruling party, the post from which he derives his power.

Deng Xiaoping, the economic reformer who was China's paramount leader throughout the 1980s and into the 1990s, was referred to as the "core" of the leadership.

But his successor Jiang Zemin was only called the core of the third generation of leaders, effectively limiting the duration of the description, Lam said. Xi's immediate predecessor Hu Jintao never achieved the status.

"A core leader is critical for a nation, for a political party," the People's Daily said in an editorial published shortly after the meeting ended.

"Formally" anointing Xi "reflects the common aspirations of the whole party, the whole military, the whole country and all of its people," it said.

The plenum confirmed that the Communist party will hold a congress late next year, when a new politburo standing committee, its most powerful body, will be selected, giving Xi an opportunity to promote favoured allies.

- Zero tolerance -

Xi's elevation comes as he exerts increasing pressure on the party to clean up its act. Since coming to power, he has presided over an anti-corruption campaign that has punished more than one million officials in what some say resembles a political purge.

Casualties have included ranged from so-called "flies", minor officials, to "tigers", major figures including top generals in the People's Liberation Army and seemingly invincible former security czar Zhou Yongkang.

The drive has eliminated potential rival bastions of power, but it has also laid waste to the party's organisational chart, paralysing grassroots bureaucrats petrified of making a mistake, a problem compounded by unclear and contradictory signals on what policies to pursue.

While some had speculated the party might use the opportunity of this week's conference to dial back its anti-graft campaign and give cadres some room to breathe, the communique suggested the opposite, calling for a "zero tolerance" attitude towards misbehaviour.

The party must "persevere in constructing a system where [party members] do not dare to be corrupt, cannot be corrupt and do not think about corruption," it said.

To that end, the communique called for strengthening the party's internal controls, including increasing ideological conformity and more strictly monitoring individual members' behaviour, the People's Daily said.

But it sought to allay concerns about overzealous implementation of the rules, allowing for forgiveness in the event that an error was made in the pursuit of "reform and innovation".