A few hours ahead of the summit in Laos, the Philippines defence ministry released pictures of Chinese boats near the disputed Scarborough Shoal in the South China Sea.
Earlier on Monday, the Philippines expressed "grave concern" about the increasing number of Chinese vessels around the Scarborough Shoal and demanded an explanation from the Chinese ambassador.
A Philippines official said on Wednesday the defence minister, who is at the summit in Vientiane, ordered the release of the photographs and a map.
Authorities emailed 10 images and a map to journalists, many of whom are in Vientiane for the ASEAN summit.
Although the area near the disputed Scarborough Shoal in the South China Sea is merely a clutch of rocks, it is extremely important to the Philippines due to the tranquil waters around it and the rich stocks of fish.
Conflicts between China and the Philippines got worse after an arbitration tribunal rejected China's vast territorial claims in the South China Sea in July. The tribunal said no single country has sovereign rights over activity in the Scarborough Shoal, which is traditional fishing ground for the Chinese, Filipinos and Vietnamese.
China, Taiwan, the Philippines, Vietnam, Malaysia and Brunei claim parts or all of the resource-rich South China Sea.
A Philippine official accompanying President Duterte said the government found it tough to explain why Filipino fishermen cannot go back and fish in the area even after The Hague's ruling.
"We won in the arbitral court, but we could not enforce it, how can we explain that to our own fishermen?" the unnamed official told Reuters.
"So, we wanted to talk to China and resolve the issue, but the situation like this is making it more difficult. The president is asking what is China's intentions in the area?"