PHNOM PENH (Yonhap News) -- South Korea, China and Japan plan to declare the official start of negotiations to forge a three-way free trade pact when their trade ministers meet on the sidelines of a regional summit in Cambodia this week, officials said Monday.

In May, the three Asian economic powers agreed to launch talks within this year toward a free trade deal. However, the prospect of negotiations was later thrown into doubt when territorial and sovereignty spats flared anew among the three historical rivals.

This week, the leaders of the three sides are gathering in Phnom Penh for a set of annual summits hosted by the Association of Southeast Asian Nations, and their trade ministers plan to meet Tuesday on the meeting‘s sidelines to declare the start of free trade talks, officials said.

The ambitious three-way pact, if realized, would create one of the world’s largest markets as South Korea, China and Japan account for 20 percent of the global gross domestic product, 17.5 percent of all global trade, and 22 percent of the global population.

This week‘s declaration is a sign that the three countries intend to move cooperation forward despite the territorial and sovereignty rows. The plan to have the trade ministers make the declaration, rather than the leaders, however, shows tensions still weigh heavily on their ties.

South Korean President Lee Myung-bak, Chinese Premier Wen Jiabao and Japanese Prime Minister Yoshihiko Noda, are attending the conferences in Cambodia. The leaders are not expected to hold a three-way summit, however, in the first time an annual summit will be skipped.

Meanwhile, President Lee and Vietnamese Prime Minister Nguyen Tan Dung agreed to work together for the early conclusion of free trade negotiations between the two countries, the presidential office said.

During their one-on-one talks, they also agreed to continue cooperation to expand trade between the two countries to $20 billion earlier than the target year of 2015, and then further to $30 billion.

South Korea and Vietnam have seen their trade volume rising sharply in recent years from $9.68 billion in 2009 to $12.98 billion in 2010 and to $18.54 billion last year.

In an effort to further boost trade, the two sides launched free trade negotiations, with the first round of talks held in September.