MANILA, Philippines â€” China cannot tow the stranded Philippine Navy vessel BRP Sierra Madre from the disputed Ayungin Shoal as this can be considered as an attack under the United States-Philippines Mutual Defense Treaty, a Filipino expert in international law said.

University of the Philippines College of Law professor Harry Roque explained that the US cannot be involved in ongoing disputes of the country with China, but the Philippine ally may act once the decrepit World War II ship is assailed by foreign military.

"China also better rethink whether it should tow-away a commissioned naval vessel," Roque said in a statement Thursday, following reports that China is offering to remove the ship serving as the Philippines' military outpost on the shoal.

"Derelict as it may be, it is subject to full sovereign immunity and any attempt to tow it away from Ayungin may finally trigger the applicability of the US-Phil Mutual Defence Treaty," he added.

Roque noted that the standoffs between Chinese and Philippine civilian vessels in the West Philippine Sea are not covered by the 1951 accord.

"But an attack against a Philippine commissioned naval vessel may be sufficient for the purpose. The result: the West Philippine Sea, unless China backs off, may trigger the biggest armed conflict in the region since the Vietnam and Indo-China conflict,' he said. The Indochina Wars, which include the American War in Vietnam, were fought from 1946 to 1989.

Roque, the first Asian to be admitted practice in the International Criminal Court, said that neither China has title over Ayungin and similar maritime features in the contested region. It cannot also grab the territories through conquest, which is a direct violation of modern international principles.

In its position paper on the issue released Thursday, China accused the Philippines of "lying" when it said BRP Sierra Madre ran aground in 1999.

"The statement released by the Department of Foreign Affairs (DFA) of the Philippines on March 14 openly stated that the vessel 'grounded' 15 years ago was actually meant to occupy the reef, which proves that the Philippine side has been lying for 15 years," the Chinese Embassy in the Philippines said in the statement.

China also demanded that the Philippines fulfills its commitment to remove the ship--a vow which the DFA denied making.

Last Sunday, Foreign Affairs Secretary Albert del Rosario announced that the country's 4,000-page written pleading has been sent to the International Tribunal on the Law of the Sea to support its arbitration case against the Asian power.