MANILA - The Hong Kong immigration office cited an unspecified "immigration reason" for denying former Philippine Foreign Affairs Secretary Albert del Rosario entry and detaining him for at least 5 hours in a holding cell at the city's airport on Friday.

The former top diplomat, known for leading the country's winning arbitration case against China over its incursions in the South China Sea, was on a business trip. A non-executive director of Hong Kong's First Pacific, Del Rosario flew to the Chinese special administrative region for a shareholders meeting.

• Hong Kong denies entry to Philippines' former top diplomat

In an interview with ANC, Philippine Consul General in Hong Kong Antonio Morales said immigration authorities did not give a clear reason or "elucidation" as to why Del Rosario was denied entry.

He said the Philippine consulate was just given "immigration regulations" for an explanation.

"I asked the immigration authorities there what was the reason for the delay in the processing of the entry, and I was told that it is for some immigration reasons. And so I contacted the head of the immigration department of the airport, and I was told the matter was being processed," Morales said.

"Aside from the immigration reason, there was no other reason given," Morales added.

The diplomat said the Philippine consulate inquired why the decision to deny Del Rosario entry took long, but they were given "no categorical answer."

Morales revealed that when they got a call from the Department of Foreign Affairs regarding Del Rosario's flight to Hong Kong hours before the his arrival, the Philippine consulate requested the Hong Kong immigration office to process the former top diplomat at the airport's courtesy lane, an express lane open to government officials and other dignitaries.

But the Philippine mission did not receive a reply from immigration authorities.

"We did not get a reply. So I was insisting that they should have given us a reply to save the former secretary the trouble of traveling into Hong Kong (if he was going to be denied entry)," Morales said.

Morales said all immigration departments around the world recognize the privilege of top officials in governments, current or former, to be given courtesy lanes and clearances.

"Generally, immigration authorities in any part of the world has this prerogative which they respect. We expected some courtesy to a former official of the government, especially since I was already there," he said.

Morales said the Philippine consulate would request a meeting with the head of Hong Kong airport's security department, which oversees the immigration office.

"I would convey our concerns with this incident," he said.

He said he respects Hong Kong's decision but that he "would appreciate being informed beforehand."

Del Rosario, who bears a diplomatic passport, reportedly arrived at the Hong Kong airport at 7:43 am and was detained. He was denied entry around 1:30 pm.

He has decried the incident as a violation of the Vienna Convention on Diplomatic Relations. Article 40 of the convention provides that a diplomatic agent is entitled to "inviolability and such other immunities as may be required to ensure his transit or return" through a territory of a third state.

Hong Kong's refusal of entry to Del Rosario came exactly a month after former Ombudsman Conchita Carpio Morales was barred from the city while traveling for a vacation with her family.

• Ex-Ombudsman Carpio Morales denied entry to Hong Kong

The immigration incidents came after Del Rosario and Carpio Morales, along with a group of Filipino fishermen, lodged a complaint of crimes against humanity against Chinese President Xi Jinping before the International Criminal Court in March over Chinese incursions in the disputed South China Sea.

Both were senior government officials during the presidency of Benigno Aquino III.

Del Rosario is also a vocal critic of the government's friendlier stance towards China despite the maritime dispute.