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MANILA (UPDATE 3) - Former Philippine Foreign Secretary Albert del Rosario, who led the country's winning arbitration case against China, said he was stuck at Hong Kong immigration on Friday waiting for clearance.

Former Ombudsman Conchita Carpio Morales, who along with Del Rosario, filed a "communication" against Chinese President Xi Jinping before the International Criminal Court, was also held for questioning at Hong Kong Airport last May.

Del Rosario confirmed to ABS-CBN News at around 9 a.m. that he had been held at immigration. He told ANC before noon that he was "detained" for nearly 4 hours.

"We are of course questioning what the reasons are, why we were detained here or held up," Del Rosario said. "There's no rational explanation."

Del Rosario, a non-executive director of Hong Kong's First Pacific, was traveling to the Chinese special administrative region for a shareholders meeting, said his lawyer, Anne Marie Corominas.

"This is pure harassment here," Corominas told ANC. Del Rosario was carrying a diplomatic passport, she said.

"He hasn’t been told of the reason why he has been detained and asked to wait," she said.

The actions of Hong Kong authorities violate the Vienna Convention because Del Rosario has diplomatic immunity, she said.

The former top envoy was instructed to stay in a small room where immigration officials asked him about the nature of his travel, Philippine Consul General in Hong Kong Antonio Morales told reporters.

Justice Secretary Menardo Guevarra, who is the current caretaker of government, said Friday he would instruct the Department of Foreign Affairs to look into Del Rosario's case and send help.

"As OIC (officer-in-charge), I will request the DFA to find out the reason for former SFA's (secretary of Foreign Affairs) exclusion and extend whatever assistance could be given to him as a former foreign minister of our country," he said.

"But personally, I believe that the lesson derived from former Ombudsman Morales' similar experience should have been clear to him," he said.

Del Rosario, who served under former President Benigno "Noynoy" Aquino III, had long accused China of bullying in the South China Sea, where Manila and Beijing have overlapping claims.

The United Nations-backed Permanent Court of Arbitration handed down its ruling in July 2016, a few days after Aquino relinquished power to incumbent President Rodrigo Duterte.

The sinking of a Filipino fishing boat by a Chinese vessel in disputed waters last June 9 is the "worst news yet," Del Rosario said in a statement earlier Friday.

“We should really feel sorry for our poor fishermen as the ultimate product of a joint probe with Beijing is expected to be no more than a bowl of fruit salad,” he added.

“Can we please not insult the intelligence of our people?” he said.

Manila initially accused the Chinese boat crew of abandoning the Filipino fishermen at sea after the incident near Recto (Reed) Bank. A Vietnamese ship rescued the Filipinos.

- with a report from Zhander Cayabyab, ABS-CBN News