By Michael MartinaMANILA, July 22 (Reuters) - U.S. Secretary of State Marco Rubio will meet Chinese Foreign Minister Wang Yi on Wednesday on the sidelines of aregional gathering led by the Southeast Asian bloc ASEAN, where the two top diplomats are expected to discuss a possible September summit between their leaders.Rubio's meeting with Wang, which was confirmed by a State Department official, comes amid a fragile truce between the global superpowers that could be complicated by U.S. President Donald Trump's accusations last week of Chinese meddling in U.S. elections, which Beijing has denied.The wider meeting in Manila takes place amid renewed hostilities and a potential new front in the conflict between the U.S. and Iran, and as diplomatic tension flares between Beijing and close U.S. ally the Philippines over an incident in the South China Sea that led to the summoning of ambassadors in both capitals.With their economies highly exposed to energy disruptions from the Middle East conflict, the foreign ministers of the Association of Southeast Asian Nations on Tuesday expressed serious concern about recent developments in the war.Meeting them on Wednesday, Rubio said the problem was that Iran was not serious about talks. "If they're serious, we're serious. If they're not, then we will do what's necessary to protect our interests, and also the interests of our allies," he said.Rubio stressed that Iran could not be allowed to control the Strait of Hormuz, arguing it could create a dangerous precedent that could be repeated in other parts of the world, including Southeast Asia.'COMMERCE AS A GEOPOLITICAL WEAPON'His remarks follow an opinion piece in Philippine media published on his arrival in Manila on Tuesday that took aim at Beijing, in which he said ASEAN countries were confronting "new and coercive threats" in the South China Sea and the U.S. was committed to upholding freedom of navigation."That freedom is by no means guaranteed. If those waters fall under the control of a power willing to use commerce as a geopolitical weapon, both the United States and the ASEAN states would face dire new threats to their sovereignty, security, and economic futures," Rubio wrote.Beijing claims sovereignty over almost the entire South China Sea via a line on its maps that cuts into the exclusive economic zones of the Philippines, Malaysia, Vietnam, Brunei and Indonesia, and is at the centre of long-running disputes over a multitude of islands and features.Wang is due to meet on the sidelines on Wednesday with Philippine Foreign Secretary Maria Theresa Lazaro, the chair of the ASEAN-led gathering.Rows between the Philippines and China have increased in recent years as President Ferdinand Marcos takes a bolder stance on the South China Sea, while pursuing closer defence ties with Washington and expanding security relationships with Australia and Japan.In talks with ASEAN's Secretary-General Kao Kim Hourn on Tuesday, Wang said certain elements of the Philippines' security apparatus had "deliberately engaged in provocations and created maritime incidents", according to Beijing's readout of the meeting.Wang said that served the interests of "external forces", adding the South China Sea issue should not be an obstacle in its relations with ASEAN and Beijing was willing to work with the bloc to "eliminate interfering factors".Australia, Russia, Japan, Canada, South Korea, Britain, the European Union and India are also attending the meetings in Manila.Analystshave saida meeting between Rubio and his Chinese counterpart would likely focus on preparations for a second meeting between Trump and Xi after the two met last May.Trump has said Xi will visit the U.S. at the end of September, and Rubio on Sunday said the summit was still anticipated. (Reporting by Michael Martina, Karen Lema, Nestor Corrales, Mikhail Flores; Writing by Martin Petty; Editing by Christian Schmollinger)