Hangzhou police have asked the local procuratorate for permission to arrest a nanny accused of starting an apartment fire that killed the three children she was employed to care for, as well as their mother.

Mo Huanjing, 34, was charged on Wednesday with arson and theft in connection with the blaze that swept through the family's apartment in a high-end residential building on June 22.

The oldest child was 11 and the youngest was 6.

The case has generated heated discussion among the public because the family had good relations with the babysitter and any motivation is still unknown.

The husband, Lin Shengbin, was on a business trip when the fire happened. He said the babysitter had borrowed money from his wife, Zhu Xiaozhen, a couple of days before.

Born in Dongguan, Guangdong province, Mo had been working for the family for about a year.

A few days before the fire, she borrowed 100,000 yuan ($14,700) from Zhu, claiming that she wanted to buy a house.

However, Mo, who police said is addicted to online gambling, didn't use the money to buy any property.

A former friend of Mo's, who went by the alias Meng Qi, said to Chinese media that Mo used to work for her, but was fired when she was reported by some employers to have stolen things.

"She took the things to pawnshops and used the money for gambling," Meng said.

After being questioned, police said, Mo admitted to having stolen watches and children's bracelets from Lin's family.

The fire started at around 5 am in the 18th-floor apartment and spread over about 50 square meters.

The young mother reportedly tried twice to call for help, but her calls were unanswered.

Zhu and her three kids were rescued from the building, but died shortly after in hospital.

Zhu's brother tried to enter the premises, but was pushed back by firefighters at the scene.

The babysitter claimed at first that she was sent downstairs by her employer to call the police, but after further investigation she admitted that she started the fire on purpose, police said.

The case has caused concern among the public about hiring babysitters as well as loopholes in background information and qualification reviews of babysitters at domestic companies.