Japanese Emperor Akihito has urged the younger generation to "keep alive the memories of the Second World War" and the hardship that followed, as a way of avoiding conflict amid growing maritime tension in the East and South China Sea.

Akihito, 82, who met Philippine President Benigno Aquino privately at the start of a four-day state visit, expressed remorse over atrocities in the region by the Japanese imperial army 70 years ago.

The emperor wanted to remind young people who had not experienced the war not to forget the hardship it brought to both Japan and other Asian countries, his press secretary, Hatsuhisa Takashima, told reporters in the Philippine capital.

"It's a thing which should not be repeated again," Takashima added. "He has a strong feeling towards war...and that's the reason he came here."

He said the Japanese emperor, who as a child had experienced the horrors of war, was worried the younger generation would not "keep alive the memories of the Second World War".

As a young prince, Akihito was sent to the mountains to escape American bombings in Tokyo. He returned after the war to find the capital in ruins.

The emperor's comments come against a backdrop of growing regional tension as China presses more assertively its claims to almost the entire South China Sea, which is believed to have huge deposits of oil and gas.

But Brunei, Malaysia, the Philippines, Taiwan and Vietnam have conflicting claims. China and Japan are contesting islands in the East China sea known as Senkaku in Japan and Diaoyu in China.

Akihito, banned by Japan's constitution from any political role, has often urged his nation not to forget the suffering of the war and tried to promote reconciliation with neighbours.

Aquino's communications secretary, Herminio Coloma, said the president and the emperor talked about his first visit in 1962, the traffic congestion brought by Japanese car sales and retail shops from Japan.

Two war-related issues – the return of the remains of more than 500,000 Japanese soldiers who died, and the sexual slavery of "comfort women" in the Philippines – would be left to the government of Japanese Prime Minister Shinzo Abe, he added.

"The emperor will not touch upon these issues by himself, it is left solely to the Abe government," Takashima said.

Akihito offered flowers in the national heroes' cemetery, and visits a Japanese war memorial south of Manila on Friday.