MANILA—The Philippines said Vietnam was considering joining its challenge to Beijing at an international tribunal, the clearest sign yet the two countries could join forces over maritime disputes with China.

Vietnam has “exhausted all dialogue channels” with Beijing, Prime Minister Nguyen Tan Dung told the World Economic Forum in Manila on Thursday. “China not only failed to respond to Vietnam’s legitimate demands, but on the contrary has been slandering Vietnam while threatening to use force,” Mr. Dung said.

Philippine Secretary of Foreign Affairs Albert del Rosario said shortly after Mr. Dung’s speech that the Vietnamese government was evaluating whether to participate in the arbitration case launched at the United Nations tribunal by Manila last year, which challenges the legality of Beijing’s claim to most of the South China Sea.

On Vietnam’s Foreign Ministry website, in response to questions from the Associated Press, the government said it was “considering various defense options, including legal actions in accordance with the international law.”

Hanoi’s strategy to beat back territorial challenges has been to build up its military rather than endorsing or emulating the Philippines’ approach of using legal means against China to protect its sovereignty. But with diplomacy failing, according to Mr. Dung’s account, Hanoi is now exploring other means of defending its interests—even at the risk of further alienating China, a crucial economic partner.