(CNN Philippines) — A local maritime expert said on Monday (April 13) that China's most recent activity in the Mischief Reef in the Spratly Islands is Beijing's response to the arbitration case filed by the Philippines.

"The reclamation was clearly a response to the arbitration. If ever China had real plans to do this before, clearly the arbitration case accelerated those plans," said Dr. Jay Batongbacal, director of the UP Institute for Maritime Affairs and Law of the Sea, during an interview with Amelyn Veloso on CNN Philippines' Headline News.

The Philippines has filed its arbitration case to the United Nations Convention on the Law of the Sea (UNCLOS) “with respect to the dispute with China over the maritime jurisdiction of the Philippines in the West Philippine Sea” last January 2013.

The country likewise submitted last March 15 a 3,000-page supplemental documentation – which includes 200 pages of written arguments and a 200-page atlas containing detailed information about 49 islands, reefs, and other features in the West Philippine Sea – rebutting Beijing’s claim that an international body such as the UNCLOS does not have the jurisdiction to decide on Manila’s pleading.

The arbitration panel, for its part, gave China three months – or until June 15 – to respond on Manila’s latest pleading. "Although we do not expect it to do so," he said.

Related: Philippines says China's reclamation causing ecological damage

Reclamation on the disputed reef have been going on since last year, with China building at a rate of one island every four months, according to Batongbacal.

China also never clearly justified its reclamation in the West Philippine Sea, other than the reasoning that it was their right to do so, he added.

Batongbacal, however, noted the change in China's reasoning in the last month. He said China tried to justify its activities as being necessary to strengthen some civilian aspects of its activities.

Earlier, Chinese Foreign Ministry spokesperson Hua Chunying said the reclamation and building work in the Spratly Islands of the West Philippine Sea would be used for military defense as well as to provide civilian services that would benefit other countries.

Related: China mounts detailed defense of sea reclamation

Batongbacal said China felt the need to explain itself to the international community as a result of constant questioning and pressure.

Geopolitical implications

China claims 90% of the West Philippine Sea is its territory.

Batongbacal said China wants to be at par with the other claimant countries, such as Vietnam, Malaysia, and the Philippines — all of which have been able to establish footholds in the West Philippine Sea.

He said that for the longest time, China didn't have the same assets of these countries, so China perceived it as a threat to their dominance in the West Philippine Sea.

"Regardless of the arbitration case, China decided to do what they felt as necessary, which was to establish their own islands," Batongbacal said.

He added that the implication of China's reclamation to the international community is the threat to their freedom of navigation and usage of the West Philippine Sea.

Last week, U.S. called for more transparency insofar as China's intention on its reclamation activities, as U.S. President Barack Obama voiced concern over China's "use of its sheer size and muscle to force countries in the subordinate positions."

Watch: Obama voices concern over China's show of force

Mischief Reef

China is currently working on reclamation activities around the Mischief Reef that is still within the Philippines' exclusive economic zone.

Mischief Reef is China's most recent reclamation in the West Philippine Sea.

Batongbacal estimated the size of the said reef as being larger than an ordinary barangay with a strategic position vital for fishing and shipping vessels.

The completely submerged and oblong in shape Mischief Reef lies 125 nautical miles away from Palawan.

In a statement issued last week, Malacañan strongly opposed China's reclamation in the disputed islands in the West Philippine Sea and reiterated the government’s commitment of pursuing legal and diplomatic tracks in resolving maritime disputes.

Watch: Philippines protests reclamation, sticks to diplomacy