The inquiry was prompted by media reports that carried a leaked document with an annotation that links the president's longtime special assistant, Christopher "Bong" Go, to the process of procuring a combat management system for the frigates from a South Korea's Hyundai Heavy Industries.

The hearing called by the Senate committee on national defense and security is looking to establish whether or not Go had gone beyond his remit by getting involved in a defense contract. He has not been accused of corruption.

Go denies any wrongdoing and read a statement critical of the media, calling it "fake news". He said he was simply passing on to the defense department a "complaint" the Office of the President had received.

The Philippine Daily Inquirer newspaper and news site Rappler last month carried leaked copies of a white paper originating from the president's office and asking the navy to look at a proposal by a South Korean subcontractor, Hanwha Land Systems, to supply the weapons system.

Defence Secretary Delfin Lorenzana has confirmed he added an annotation to that document telling the navy chief to look at the proposal, and that the document had been given to him by Go.

However, Lorenzana told the inquiry on Monday that he could not be certain who gave it to him.

Hanwha Land Systems in South Korea did not immediately respond to a request for comment and was not represented at the hearing.

Sandra Han, a representative for Hyundai Heavy Industries, attended Monday's hearing but was not asked about whether Go was involved or not. A Hyundai Heavy Industries communications official could not immediately be reached for comment.

Go said his office had merely endorsed what he said was a complaint from Hanwha. He did not specify what the complaint was about.

"We are being castigated for endorsing a complaint in (to) the proper agency. A mere notary endorsement, which is one of the thousands of complaints we endorse," he said, adding that it was Duterte's agenda to be open to receive complaints about state officials.

He said the frigate controversy was "seriously derailing" the implementation of an important security program and was cooked-up by the administration's opponents.

"We will not allow them to win because the truth is on our side," he said, attracting applause from the hearing.

Duterte's spokesman, Harry Roque, said the contract, which was negotiated during the administration of Duterte's predecessor, Benigno Aquino, underwent scrutiny by the defense department under the Duterte administration.

"The conscience of the Duterte administration is very clean," Roque said on radio ahead of the hearing.

"We implemented, we followed the law, but it was not this administration which chose the supplier."

Senator Antonio Trillanes, one of Duterte's staunchest critics, believed Go would not do anything without Duterte's order.

"If it is established that he was involved, then it is not because Bong Go had an interest in it, but rather Duterte had an interest in it," Philippine Star newspaper quoted Trillanes as saying.

Vice Admiral Ronald Joseph Mercado was sacked as navy chief in December for insisting those ships be installed with combat systems made by a Dutch company.

Mercado told the hearing that said he had at no point spoken to Go about the weapons system.