"An ADIZ would be a catastrophe. It would severely limit air and maritime activity," a senior Japanese policymaker told Reuters on condition of anonymity.
New boats, exercises
The planned joint naval exercises off the Philippines are part of a security agreement signed in Tokyo in January, which also outlined regular vice-ministerial defense talks and exchanges of senior officers.
The first of 10 coastguard vessels Japan is building for the Philippines will be delivered by the year-end.
Japan might also fund infrastructure improvements around a Philippine military base on Palawan Island, one of the closest major land masses to the Spratlys, said a Japanese source with knowledge of the plan.
The Philippine armed forces spokesman, Colonel Restituto Padilla, applauded Tokyo's engagement, adding it would be "natural for Japan and the Philippines to join forces to help each other secure these sea lanes".
Japan has not gone that far, although Defense Minister Gen Nakatani said last month that a policy of not sending patrol aircraft over the South China Sea might need to be reviewed given the importance of the waterway to Tokyo.
Nakatani's comments came after a top U.S. Navy officer said Washington would welcome Japanese air patrols in the region.
Vietnam's government did not immediately comment on Japan's assistance, which will include six used navy patrol boats for Hanoi as well as advice on treating decompression illness as Vietnam takes possession of additional sophisticated submarines from Russia.
"There are a lot of other things the Vietnamese want," said a Japanese Defense Ministry source without elaborating.
Elsewhere, Japanese Defense Ministry officials have held very preliminary talks on joint weapons production with Malaysia and Indonesia, the sources said.
Abe has also been drawing closer to Australia.
Canberra recently sent a Defense Ministry official to Tokyo to help Japan build ties in Southeast Asia, the Japanese Defense Ministry source said. An Australian defense spokesperson said an official had been posted to the Japanese ministry for 18 months, but did not give details on the person's role.
Changes to Japan's defense posture in the coming months will allow Abe to further deepen his involvement in Southeast Asia. For example, Japan is mulling military aid to finance arms sales.