JAKARTA (AFP) - A summit of Southeast Asian leaders focusing on economic integration took a dramatic turn Saturday when Cambodian Prime Minister Hun Sen blasted his Thai counterpart over a bloody border dispute. Hun Sen took the floor of the closed-door opening session of the two-day summit in Jakarta and accused Thailand of attacking its neighbour in a contested border area near an 11th-century Khmer temple, attendees said. One foreign minister described the fiery Cambodian leader as "quite aggressive" and observers said the assembled Association of Southeast Asian Nations (ASEAN) leaders were surprised at the outburst. "It became a little dramatic, but I think that's just the way that Prime Minister Hun Sen delivers speeches," Philippine presidential spokesman Ricky Carandang said. Thai Prime Minister Abhisit Vejjajiva told reporters after the encounter: "We had a frank discussion this morning." "We need to resolve the problem because we don't want this to be a problem that would affect ASEAN 's agenda on community building," he added. The border spat was not on the formal agenda of the two-day summit but it is overshadowing discussions on ASEAN 's long-term efforts to create a closely integrated regional economic zone by 2015. Other issues on the table include food and energy security, territorial disputes in the South China Sea, the scourge of human trafficking and East Timor's membership bid. But even before the presidents and prime ministers of the disparate 10-nation bloc sat down in Jakarta's cavernous convention centre, their discussions had been framed by negative news from troubled member states. Myanmar stole the headlines on Friday when ASEAN officials announced that the military-led country - which is under Western sanctions for serial human rights abuses - had asked to chair the group in 2014. US-based Human Rights Watch said ASEAN , already struggling for credibility, would become the "laughing stock of intergovernmental forums" if it granted the request. ASEAN leaders are meanwhile facing mounting pressure to help end the Thai-Cambodian conflict, which has claimed 18 lives and temporarily displaced 85,000 people in weeks of clashes. Indonesian President Susilo Bambang Yudhoyono spoke of the dangers of rising food and energy prices, climate change and unrest in the Middle East during his opening remarks. But without mentioning the Thai-Cambodian row, he also reminded his fellow leaders that ASEAN could no longer use the bloc's long-held principle of non-interference to shirk the need to resolve conflicts between member states. "We realise that to ensure a peaceful and stable East Asia region, we must ensure stability and security in our region," the ex-general said. "If conflict occurs, ASEAN must be capable of facilitating a forum for diplomacy and open dialogue with the intent of attaining common peace." ASEAN 's halting efforts to negotiate an end to the clashes are being closely watched as a litmus test of its soaring ambition to create an integrated regional community in just four years' time. Indonesian Foreign Minister Marty Natalegawa met his counterparts from Thailand and Cambodia in Jakarta on Friday and said they had agreed to accept Indonesian military observers at the disputed frontier. But he said the modest observer mission, which would have no power to police a ceasefire, had yet to be dispatched because of stubborn differences over troop locations. Cambodian Foreign Minister Hor Namhong told reporters that Phnom Penh would never agree to Bangkok's demands to pull troops out of the Preah Vihear temple. "Prime Minister Hun Sen appealed to ASEAN to help solve the problem peacefully," he said. Thai Foreign Minister Kasit Piromya said there was little enthusiasm among ASEAN leaders for the conflict to be internationalised with direct UN involvement, as Cambodia has sought. "I think what all the other ASEAN leaders have been saying this morning is that we should keep the conflict within the ASEAN family," he said. UN Secretary-General Ban Ki-moon and US Secretary of State Hillary Clinton have publicly backed ASEAN 's mediation role. ASEAN groups Brunei, Cambodia , Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Vietnam.