BEIJING (AFP) - China's state media called on Friday for "timely countermeasures without hesitation" if Japan violates the country's newly declared air zone, after Beijing sent fighter jets to patrol the area following defiant military overflights by Tokyo.

Japan and South Korea both said on Thursday that they had disregarded the air defence identification zone (ADIZ) that Beijing declared last weekend, showing a united front after US B-52 bombers also entered the area.

The zone includes disputed islands claimed by China, which knows them as the Diaoyus, but controlled by Japan, which calls them the Senkakus, and Beijing's ADIZ was condemned in Washington, Tokyo, South Korea and elsewhere.

China sent fighter jets and an early warning aircraft into the area on Thursday, the official Xinhua news agency reported after Tokyo said its military and coastguard had both flown through it.

Washington has security alliances with both Tokyo and Seoul, and analysts say that neither China nor Japan - the world's second- and third-biggest economies, and major trading partners of each other - want to engage in armed conflict.

But Beijing is facing mounting internal pressure to assert itself.

The Global Times newspaper, which often takes a nationalistic tone, said in an editorial on Friday: "We should carry out timely countermeasures without hesitation against Japan when it challenges China's newly declared ADIZ.

"If Tokyo flies its aircraft over the zone, we will be bound to send our plane to its ADIZ."

The paper, which is close to China's ruling Communist party, raised the prospect of "tension in the air like in the Cold War era between the US and the Soviet Union", adding: "We are willing to engage in a protracted confrontation with Japan."

But it shied away from threatening Washington, which sent giant Stratofortress bombers inside the zone, issuing an unmistakable message.

"If the US does not go too far, we will not target it in safeguarding our air defence zone," the paper said, adding that Australia could be "ignored" and that South Korea "understands" as it has tensions of its own with Japan.

China's Communist Party seeks to bolster its public support by tapping into deep-seated resentment of Japan for its brutal invasion of the country in the 1930s.

Such passions are easily ignited, and posters on Chinese social media networks have urged Beijing to act, with one saying on Friday: "Japan, US and South Korea jointly ran the red light on purpose, the Chinese government should take decisive measures to deal with them as a war of words won't solve any problems."

The media rhetoric came after Chinese planes conducted air patrols on Thursday, Xinhua said, after it first sent fighters, scouts and early warning aircraft into the zone on Saturday.

The Chinese ADIZ requires aircraft to provide their flight plan, declare their nationality and maintain two-way radio communication - or face unspecified "defensive emergency measures".

Both Japan and Washington have ADIZs of their own, and China accuses them of double standards, saying the real provocateur is Tokyo.

Japan denies that there is a dispute over the islands, and Chief Cabinet Secretary Yoshihide Suga declined on Friday to be drawn into discussing reports that a Chinese envoy had suggested setting up a mechanism to prevent mid-air incidents.

"Our country's principle is that we will assert our position firmly in a stern but calm manner," Mr Suga said. "And we keep the window of dialogue open."

The Yomiuri Shimbun reported that the US and Japan planned to enhance military co-operation in the area, with Tokyo permanently stationing E-2C early-warning planes in Okinawa, and US Global Hawk unmanned drones expected to be operated from Japan soon.

The European Union added its voice to the criticism of the zone on Friday, with its top foreign affairs official Catherine Ashton saying it "contributes to raising tensions in the region".

At a regular briefing, Chinese foreign ministry spokesman Qin Gang retorted: "Ms Ashton knows that within the EU, some countries have ADIZs, so I don't know if that means the situation in Europe is getting more tense."

US Vice-President Joe Biden is visiting the region next week, and administration officials said that while in Beijing he will raise Washington's concerns about the ADIZ, and China's assertiveness towards its neighbours.

The Philippines has voiced concern that China may extend control of air space over disputed areas of the South China Sea, where the two nations have a separate territorial dispute.

The islands dispute lay dormant for decades but flared in September 2012 when Tokyo purchased three of the uninhabited outcrops from private owners.

Beijing accused Tokyo of changing the status quo and has since sent surveillance ships and aircraft to the area, prompting Japan to scramble fighter jets hundreds of times.