Duterte said he was making the threat public in response to domestic criticism he was being too weak with China over the dispute.
Duterte, who met with Chinese President Xi Jinping and Premier Li Keqiang in Beijing this week, said he was making the threat public in response to domestic criticism he was being too weak with China over the dispute.
"I really said it to their face. That is ours and we intend to drill oil there," Duterte said, revealing his previously unreported plans to explore for natural resources in disputed areas.
"And they told me: 'Well, we're friends. We do not want to quarrel with you. We want to maintain the present warm relationship. But if you force the issue we'll go to war.'"
The competing claims to the sea, which is believed to sit atop vast oil and gas deposits, have for decades made it one of Asia's potential military flashpoints.
China claims most of the sea, a key waterway for global shipping, and has reclaimed disputed reefs and installed military facilities on them.
Malaysia, Brunei, Vietnam and Taiwan also have overlapping claims.
An international tribunal ruled in July last year that China's claims to most of the sea were without legal basis, in a case filed by the Philippines under Duterte's predecessor, Benigno Aquino.
But China vowed to ignore the ruling and warned the Philippines against trying to use the verdict as leverage.
Duterte, who began his six-year term in June last year, agreed to take a soft stance with China, claiming that if he did it might lead to war.
Duterte has also sought closer ties with China to win billions of dollars of Chinese investments and loans, while loosening the Philippines' long-standing alliance with the United States.
That change of tack earned Duterte a state visit to China in October last year, when he met with Xi.
Duterte returned to China this week to take part in a summit on a Chinese plan to expand its trade and infrastructure links around the world.
On his return to Manila on Tuesday Duterte said he was open to exploring the sea's natural resources with China and Vietnam, but did not mention the other claimants.
Duterte also reiterated on Tuesday he had no immediate plans to pressure Beijing over the arbitral tribunal's ruling.