VIENTIANE: The Philippines' defense ministry released pictures on Wednesday showing what it said were Chinese boats near a disputed shoal in the South China Sea, just hours before Southeast Asian nations were due to meet China's premier at a summit in Laos. There was no explanation for the timing of the release, but it came two days after Manila expressed 'grave concern' about the increasing number of Chinese vessels around the Scarborough Shoal and demanded an explanation from Beijing's ambassador. A Philippines official said the release of the photographs and a map was ordered by the defense minister, who is at the summit in Vientiane, Laos. China, Taiwan, the Philippines, Vietnam, Malaysia and Brunei claim parts or all of the resource-rich South China Sea, making it a hotspot of regional tension. The last four are members of the Association of Southeast Asian Nations (ASEAN). The 10 images and map were sent by email to journalists, many of whom are in Vientiane for the ASEAN summit. The leaders were due to meet Chinese Premier Li Keqiang on Wednesday, although it was unclear if the row over the South China Sea would be openly addressed. The move by the Philippines comes after a spat with the United States, its main ally. Philippines President Rodrigo Duterte criticised US President Barack Obama, prompting the cancellation of a meeting between the two leaders in Laos. China has repeatedly blamed the United States for stirring up trouble in the South China Sea, a strategic waterway through which more than $5 trillion of trade moves annually. The United States says it has no position on the territorial dispute, but has conducted freedom of navigation patrols close to Chinese-held islands, to Beijing's anger, while China has been bolstering its military presence there. FEW ROCKS Although the Scarborough Shoal is merely a few rocks poking above the sea, it is important to the Philippines because of its tranquil waters and rich stocks of fish. Manila says China's blockade of the shoal is a violation of international law. The dispute has become more significant since the Permanent Court of Arbitration ruled on July 12 that no one country had sovereign rights over activity in the Scarborough Shoal, a traditional fishing ground for Chinese, Filipino and Vietnamese. China has refused to recognise the ruling by the court in The Hague. Duterte wants China to abide by the ruling but he had pledged not to raise the issue during the meeting in Laos. He wants to smooth the way for bilateral negotiations and last month sent former President Fidel Ramos as his special envoy to meet Chinese representatives in Hong Kong. A draft ASEAN communique seen by Reuters on Monday listed eight points related to the South China Sea, but made no mention of the ruling. However, Duterte's defense minister said ahead of the summit that a Philippines air force plane had flown over the shoal and spotted more boats than usual in a flotilla China has maintained since seizing the shoal after a tense standoff in 2012. Defence Secretary Delfin Lorenzana said the presence of six Chinese vessels in addition to coastguard ships in the area was 'a cause of grave concern'. A Philippine security official traveling with Duterte said it was a challenge for the government to explain why Filipino fishermen cannot go back and fish in the area when The Hague had ruled that Scarborough was a fishing ground for all. 'We won in the arbitral court, but we could not enforce it, how can we explain that to our own fishermen?' said the official, who declined to be named. '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?'