In its campaign to win international support for its position, China has focused mostly on small nations that depend on Chinese trade or aid, with limited success. On April 13, China’s state-run Xinhua news agency reported that Fiji had declared its support for China’s position, which holds that any disputes countries have over the sea should be handled bilaterally. Fiji’s Information Ministry quickly replied, saying the Pacific island nation did not back China’s stance and took no sides in the matter, the Fiji Broadcasting Corporation reported.

China also claims the support of Brunei, Cambodia and landlocked Laos. In April, Russia’s foreign minister, Sergey V. Lavrov, meeting his counterpart in Beijing, said that outside nations should not interfere in the South China Sea, a reference to the United States. The United States has been challenging what it considers China’s excessive maritime claims by sailing warships close to China’s artificial islands, most recently on Tuesday, when a guided missile destroyer passed within 12 nautical miles of Fiery Cross Reef.

Euan Graham, director of the International Security Program at the Lowy Institute for International Policy in Sydney, Australia, said the court might issue a ruling that did not address the legality of China’s nine-dash line, which was first put forth in the years before the Chinese Communist Party’s victory in 1949. The court may rule that the shoals and outcroppings claimed by China are not islands, meaning that China cannot make the case that it should have exclusive economic zones around them, limiting its jurisdiction in the waters of the South China Sea, he said.

“It is likely to go predominantly the Philippines’ way,” Mr. Graham said by telephone. “That explains why China is rolling out in a very communist way this propaganda barrage.”