By Idrees Ali and Phil Stewart

WASHINGTON (Reuters) – Two senior Republican U.S. senators have asked President Joe Biden to provide a complete list of options developed by the Pentagon and State Department to help support the Philippines in recent standoffs with Beijing over a contested shoal in the South China Sea.

Central to tensions is the disputed Second Thomas Shoal, where the Philippines maintains a rusty warship manned by a small crew that it deliberately grounded in 1999 to reinforce its maritime claims. It regularly sends supply missions to troops stationed there.

Tensions in the disputed waterway have boiled over into violence in the past year, with a Filipino sailor losing a finger in a June 17 clash that Manila described as “intentional-high speed ramming” by a Chinese coast guard vessel.

The United States is bound by a seven-decade-old mutual defense treaty to defend the Philippines against an armed attack on its aircraft, or public vessels, in the busy waterway.

“We must respond with visible and concrete demonstrations of our support. Anything short of this risks our appearing unwilling to honor our bilateral commitments,” read a July 12 letter to Biden, seen by Reuters, from Senators Roger Wicker and Jim Risch, the highest ranking Republicans on the Senate Armed Services Committee and Senate Foreign Relations Committee.

“We request your administration provide us with a full list of military, diplomatic, and economic options developed by the Departments of State and Defense to support the Philippines and deter further escalation by the PRC (People’s Republic of China),” the letter said.

Philippines President Ferdinand Marcos Jr has said Manila needs to do more than protest China’s “illegal action” against its navy, without elaborating.

The Philippines has not asked the United States for support in resupplying its troops, according to its ambassador in Washington. The United States is providing some limited intelligence support to the Philippines.

The South China Sea, vital to global trade, has become a major flashpoint in the testy relationship between China and the United States.

The senators’ letter said the Biden administration must move quickly to support the Philippines “in countering China’s aggressive behavior …. Limiting our response to verbal assurances of the applicability of Article IV undermines the credibility and value of these commitments.”