Hoang Lam-phuc, 17, and Liu Tsz-man, 18, were earlier charged with attempted arson and possession of weapons. They could now face an additional charge of absconding.

The two - 16 and 17 when caught off the mainland - were escorted back to Hong Kong after the Yantian People's Procuratorate said they would not face illegal border crossing charges.

Cheng Lai-ki, an acting chief superintendent in the Commercial Crime Bureau, said as the two missed court hearings in Hong Kong there were warrants for their arrests.

"The courts will decide whether to continue to remand them," Cheng added. "If that's the case they might have to spend their 14-day quarantine in the Correctional Services Department."

Tsuen Wan District Councilor Lester Shum Ngo-fai said families had a short meeting with Hoang and Liu, who seemed to have "dull expressions."

He added: "The two refused to talk about their detention in the past four months."

Shum also said Hong Kong police only allowed lawyers who had been following the cases in Hong Kong to visit at first.

"The practice [of allowing just family-appointed lawyers] to meet detainees only appears in the mainland's judicial system," he said. "We felt astonished at the Hong Kong police arrangement."

Activist Owen Chow Ka-shing, who has assisted families of the 12 Hongkongers, said: "It's obvious the detainees were physically and mentally abused in custody" in Yantian.

Hong Kong police arrested Liu with four others on September 30 last year after they received a report someone was making petrol bombs inside a flat on Lockhart Road.

Hoang was charged with attempted arson with intent and possession of offensive weapons. He was arrested by the police on October 14 last year with another male suspect for throwing petrol bombs and setting a fire at Mong Kok Police Station.