In his remarks at the turnover of USA missiles in Manila, O'Brien cited the Trump administration's role in the defeat of the Islamic State group in the Middle East and last year's killing of its leader, Abu Bakr al-Baghdadi, in Syria, and renewed its commitment to help defeat IS-linked militants in the southern Philippines.
The $18 million donation was the result of the phone call between U.S. President Donald Trump and President Rodrigo Duterte in April, and included precision-guided munitions designed for use in counterterrorism operations.
Aside from counterterrorism support, O'Brien said the U.S. will continue to be an ally when it comes to maritime security, reiterating America's recognition of the 2016 arbitral tribunal ruling on the South China Sea that negated China's historic and economic claims to much of the disputed waters.
"They belong to the Philippine people". "They don't belong to some other country just because they may be bigger than the Philippines".
"We stand with the Philippines in protecting your sovereign rights, your offshore resources, and all of those rights that are consistent with global law", O'Brien said.
O'Brien, a lawyer, took a hardline stance against China's incursions in the South China Sea. Last month, the Philippines announced that it would resume oil and gas explorations in and around Reed Bank, which lies off the coast of Palawan island and is also claimed by China. "We don't want to go back to an era of 'might makes right, '" he added.
China claim to 90% of the South China Sea includes areas claimed by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.
Duterte announced the cancellation of the agreement in February after the U.S. Congress moved to impose sanctions on top Philippine officials accused of human rights abuses.
Mr O'Brien's surprise tour of Vietnam and the Philippines reaffirmed Washington would back the two nations in maritime disputes on Beijing's allegedly "incredibly aggressive" actions in the South China Sea.
A spokesperson from the Chinese embassy in Tokyo said in October that the United States should abandon its "Cold War mentality and ideological prejudice" and treat regional relations with China "in a constructive manner".
"I think when we send that message - that peace-through-strength message - is the way to deter China".
O'Brien also welcomed Duterte's recent decision to extend the suspension of the abrogation of the Visiting Forces Agreement (VFA), which governs the deployment of US personnel on Philippine soil.
He expressed hope for the continuance of a key security agreement that allows American forces to train in large-scale combat exercises in the Philippines. The 1998 deal includes provisions on visa and passport policies for American troops, and rights of the United States government to retain jurisdiction over its military personnel, among others.