The investigation into the so-called "gentleman's agreement" between former president Rodrigo Duterte and Chinese President Xi Jinping concerning the hotly contested West Philippines would be a top priority of the House of Representatives, a House leader said Tuesday.

"The Committee on Rules has not yet determined which committee the resolution would go to to start with the inquiry… But definitely it's going to be a priority," Assistant Majority Leader Zia Alonto Adiong said in a press conference.

He, however, failed to provide a timetable on the probe, which PBA Partylist Rep. Margarita Nograles deemed "unfair" if it were to be disclosed without diving first into the legitimacy of the matter.

"As a lawyer, you have to see, and you cannot give a timeline and you cannot determine firsthand before you hear things. Right now, what we are hearing again on media and whoever is saying this and that, they're merely allegations," said Nograles, also an assistant majority leader.

"We work with facts, we work with things that are backed up with evidence and not just mere fake news, hearsay, false evidence. We look into everything and we are not bullies here. We respect anyone who would ever be invited in the committee. So, let's not [pre-empt]," Nograles added.

The House formally announced its intent to ferret out the truth behind the supposed Duterte-Xi deal during the resumption of the session on Monday following a month-long recess.

Speaker Martin Romualdez, who gave the ultimatum, said that the House would not summon Duterte yet, but the attendance in the probe of the former president's Cabinet members, including Department of Foreign Affairs officials, the executive secretary, and former spokespersons, is a must.

House Committee on the West Philippine Sea vice chairperson Mohamad Khalid Dimaporo is one with Romualdez.

"As far as inviting the former President you know, I think that's up to the Chairman. But we also have our protocol when it comes to treating our leaders and former leaders with respect," Dimaporo said in the same press conference.

However, he said, "I'd like to hear what Secretary [Delfin] Lorenzana has to say. I'd like to hear what Secretary [Hermogenes] Esperon has to say and Secretary [Eduardo] Año which is National Security, DFA also, so that the Filipino people through our House of Representatives can get a gauge on who is telling the truth in regard to these gentlemen's agreement."

Duterte denied having a secret deal with China but admitted to agreeing to maintain a status quo in the contested waterway, in which no armed patrols would be seen moving in the area to avoid tension and war.

The alleged pact is also said to have prevented the Philippines from transporting construction materials to repair and maintain the BRP Sierra Madre, a grounded Navy ship serving as a Philippine outpost in Ayungin Shoal, in the WPS.

China has territorial claims in nearly the entire South China Sea, which overlaps the WPS, notwithstanding a 2016 arbitral ruling that deemed its sovereignty assertion baseless.

The ruling affirmed the Philippines' 200-nautical mile exclusive economic zone in the WPS.