By Ben Blanchard

BEIJING (Reuters) - China's ruling Communist Party said on Saturday that the mayor of the major northern city of Tianjin was being investigated on suspicion of corruption, the latest senior official to be caught up in a war on deep-seated graft.

Dozens of senior people have been investigated or jailed since President Xi Jinping assumed power almost four years ago, vowing to go after corruption and warning, like others before, the problem threatens the party's grip on power.

In a brief statement, the party's graft-busting Central Commission for Discipline Inspection said Huang Xingguo, 61, was suspected of "serious discipline breaches", using the party's normal euphemism for corruption.

It provided no other details. It was not possible to reach Huang for comment and unclear if he had retained a lawyer.

Huang, who is also Tianjin's acting Communist Party chief, became mayor in 2008.

Tianjin is an important port city about an hour from Beijing by car that has ambitions to become a financial hub for northern China.

It is one of four conurbations - along with Beijing, Shanghai and Chongqing - termed a municipality, giving it the same high status as a province.

In August last year, a series of massive explosions at a chemicals warehouse in Tianjin killed about 170 people, sparking anger nationwide that it had been built so close to residential areas.

Last month, the party announced it was investigating Yin Hailin, a long-time city planning official who became Tianjin's deputy mayor in 2012, also on suspicion of corruption.

Huang's fall from grace appears to have come out of the blue.

On Saturday, the official Tianjin Daily covered his visit to a middle school on its front page and praised teachers for their work.

Critics have accused Xi of using the corruption campaign as a cover to root out political rivals, though he has denied this.

Last year, a Chinese court jailed former domestic security chief Zhou Yongkang for life for bribery, leaking state secrets and abuse of power.

He was the most senior Chinese official to be ensnared in a graft probe since the party swept to power in 1949.

(Reporting by Ben Blanchard; Editing by Kevin Liffey)