The Philippines has long been a staunch U.S. ally and the pact is widely seen as important for both as the Philippines confronts an assertive China in the disputed South China Sea and the United States implements a "pivot" back to Asia.

But left-wing Philippine politicians and others have challenged the constitutionality of the pact, known as the Enhanced Defence Cooperation Agreement (EDCA), which as a result, has become held up in the Supreme Court.

The deal would enable U.S. troops to build facilities to store equipment for maritime security in addition to having broad access to Philippine bases.

Last month, after the court failed to rule on it as expected, a court source said the 15-member bench was likely to uphold the constitutionality of the agreement at its Dec. 16 meeting.

But court spokesman Theodore Te declined to comment on the pact when he briefed reporters on Wednesday.

Another court official, who declined to be identified, said some judges had asked for more time to consider the opinions of those who argue the deal must be ratified by senators because it is a treaty, not an executive agreement.

The court will convene again on Jan. 12.

The pact was signed days before U.S. President Barack Obama visited the Philippines in April 2014.

Philippine military officials say there has been an increase in U.S. exercises, training and ship and aircraft visits in the past year under Obama`s "rebalance" to Asia but the pact would take the relationship a step further.

On Monday, the U.S. Pacific Fleet Commander warned of an arms race in the South China Sea as nations become increasingly tempted to use force to settle disputes.

Patrick Cronin of the Washington-based Center for a New American Security said the EDCA was essential to the U.S. rebalance to Asia.

A ruling against it would be "a major setback to upgrading the Philippine armed forces and rejuvenation of the alliance with the United States", Cronin told Reuters.

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