MANILA, Philippines (AP) — The Philippine defense secretary said Wednesday that an anchored Filipino fishing boat sank in the disputed South China Sea after being hit by a suspected Chinese vessel which then abandoned the 22 Filipino crewmen.

Defense Secretary Delfin Lorenzana called for an investigation of the June 9 sinking at Reed Bank off the western Philippine province of Palawan and asked that diplomatic steps be taken to prevent a repeat of the incident.

It's a delicate development in the long-contested waters, which are regarded as a potential flashpoint in Asia. Tensions have escalated in recent years after China transformed seven disputed reefs into islands which can serve as forward military bases and can intimidate rival claimant states.

The Philippine coast guard said it was checking whether Chinese fishermen were involved or those from other neighboring countries like Vietnam and if the collision was intentional.

There was no immediate comment from Chinese officials.

The 22 Filipino crewmen of the sunken F/B Gimver 1 were rescued by a Vietnamese vessel. A Philippine navy frigate which was patrolling the area later helped secure them, Lorenzana said in a statement.

"We condemn in the strongest terms the cowardly action of the Chinese fishing vessel and its crew for abandoning the Filipino crew," Lorenzana said. "This is not the expected action from a responsible and friendly people." He said the F/B Gimver 1 had been anchored "when it was hit by the Chinese fishing vessel."

Lorenzana thanked the Vietnamese crew for saving the Filipinos.

He revealed the incident after about 300 protesters burned a mock Chinese flag and yelled anti-China slogans in a rally outside the Chinese Consulate in Manila's Makati financial district. The mostly left-wing activists timed their protest for Philippine Independence Day.

A regional military spokesman, Lt. Col. Stephen Penetrante, said the incident at Reed Bank, which happened at night, appeared "like a hit and run," with the vessel immediately moving away after hitting the Filipino boat.

There has been a recent history of Chinese ships blocking Philippine military and civilian vessels at Reed Bank and nearby Second Thomas Shoal, where Philippine marines keep watch on board a long-marooned Philippine navy ship while being constantly watched by Chinese coast guard ships in a years-long standoff.

A Filipino official said a Philippine vessel on its way to provide the marines at Second Thomas Shoal with food and other supplies was approached by a Chinese ship "in a close encounter" in February. The Philippine vessel maneuvered to avoid the Chinese ship and managed to reach the marines, said the official, who spoke on condition of anonymity because he wasn't authorized to speak publicly. The Philippines has raised its concern over the incident, the official added.

China's coast guard and military tried to block such resupply missions in the past but later allowed them through after talks with Philippine officials amid better relations between Beijing and Manila under current President Rodrigo Duterte.

Chinese authorities, however, still occasionally approach Philippine resupply vessels to make sure they're not carrying construction materials to the disputed shoal, the Philippine official said.

China has long demanded that the Philippines remove the rusting navy ship which Filipino marines use as an outpost, but the Philippines has refused.

In 2011, the Philippine military deployed a bomber plane and another light plane to Reed Bank after a Philippine ship searching for oil complained it was approached and harassed by two Chinese patrol boats.

The patrol boats had left the area by the time the Philippine aircraft arrived, military officials said at the time.

Aside from its potential oil and gas deposits, the disputed region has rich fishing grounds and straddles busy sea lanes that are a crucial conduit for oil and other resources fueling Asia's bustling economies.