MANILA (REUTERS) - The Philippines relieved its navy commander of duty on Tuesday (Dec 19), four months ahead of his retirement, with the armed forces offering no explanation for his removal.

Military spokesman Colonel Edgard Arevalo confirmed to reporters that Vice-Admiral Ronald Joseph Mercado was no longer in charge of the 23,000-strong naval forces, but declined to say why.

"The reason for this change of command will be explained in due time," Arevalo told reporters.

Military sources quoted in media reports said Mercado's removal was connected to controversy that had surfaced over the procurement of two frigates from South Korea, which are due to be delivered by 2020.

A defence official privy to the decision to remove Mercado said it concerned equipment to be purchased for those frigates.

The official told Reuters the former navy chief had lost the "trust and confidence" of Defence Secretary Delfin Lorenzana.

Lorenzana did not respond to a request for comment on Mercado's removal and calls to Mercado's mobile phone went unanswered.

"There were some policy differences between the defense department and the navy over the 18 billion pesos (US$358 million) acquisition of two brand-new frigates from South Korea," the military official said, requesting anonymity because of the sensitivity of the issue.

Rear-Admiral Robert Empedrad was promoted to navy commander at a low-key ceremony at the armed forces headquarters in Manila.

Congressman Gary Alejano, a former marine officer, said he would request a formal congressional inquiry take place over the navy's frigate acquisition project, having found Mercado's sudden removal "unusual".

The Philippines is in the midst of a five-year, 125 billion peso (US$2.5 billion) plan to modernise its ill-equipped armed forces, acquiring new boats, planes, helicopters, rifles, radars and communication equipment.