(Refiles to clarify government statement was reported by public broadcaster RTHK)

HONG KONG, Sept 14 (Reuters) - Hong Kong's government stood fast in its refusal to interfere with the arrest of 12 residents seeking to flee to Taiwan by sea, despite pleas from families for assistance, saying the crime falls under mainland Chinese jurisdiction.

In a statement late on Sunday cited by public broadcaster RTHK, Hong Kong authorities said they had received requests for help from the families of the residents who were detained last month by mainland law enforcement for illegal entry into mainland China after trying to flee to Taiwan.

China on Sunday labelled the group as 'separatists'.

"The relevant crime falls within the jurisdiction of the mainland and the special administrative region government respects and will not interfere with law enforcement actions," RTHK quoted Hong Kong's government as saying.

The group was suspected of committing "various criminal offences" in Hong Kong, it added, as it urged the families to make use of a free legal consultation service being provided.

The comments came a day after relatives of the detainees held a news conference in Hong Kong, demanding the urgent return of the 12 who were intercepted by the Guangdong coast guard on Aug. 23 on a boat bound for Taiwan.

Donning masks and hats to shield their identities, families plead for those arrested to be allowed to consult lawyers appointed by them and not the Chinese government, and to be allowed to call relatives in Hong Kong.

A boy aged 16 is the youngest being held and several need medication, relatives said.

The arrests came about two months after Beijing imposed a security law on the Asian financial hub following months of pro-democracy demonstrations.

Critics have said the law has pushed the former British colony onto a more authoritarian path.

China's foreign ministry said on Sunday that the arrested people were separatists, in response to its U.S. counterpart's characterisation of the arrest as a deterioration of human rights.

U.S. State Department spokeswoman Morgan Ortagus had tweeted the arrests were "another example of the deterioration of human rights in Hong Kong", and called on mainland authorities to "ensure due process". (Reporting by Farah Master; Editing by Michael Perry)