Afghan police have arrested four brothers who threw acid over a woman who had refused to marry one of them, also attacked her mother and two sisters with acid, and brutally beat her father, the interior ministry said on Friday.

Seventeen-year-old Mumtaz, the eldest of three daughters, had been pursued for two years by a local gunman considered a troublemaker by the family.

Six months ago, with her parents' support, she turned him down and got engaged to a relative, but shortly before the wedding, a group of gunmen burst into their home and attacked the whole family.

This is the worst kind of brutality against women and the most critical punishment has to be given to them, said Sediq Sediqi, a spokesman for the interior ministry.

Mumtaz was the most severely injured and is being treated in India. Her mother and two sisters are in a hospital in Kabul.

Her father, who was beaten, has been released from hospital. He still has severe back and head problems, and is worried that he cannot afford to stay with his family in Kabul for long.

Police in the northern city of Kunduz detained three of the brothers on Thursday when they tried to arrange a meeting of elders to seek forgiveness from the family. The fourth was arrested on December 1.

Kunduz police are now seeking to arrest two or three more people the family has accused of attacking them, Sediqi said.

Very soon it will be clear how many people were involved in the case, he said.

According to Sediqi, the four detainees are accused of entering the house, beating the family and pouring acid, carrying an illegal weapon and stealing 50,000 Afghanis.

The family told police that six or seven armed men burst into their home in the Bulk Awal area of Kunduz -- the largest city in the region -- in the middle of the night on November 30.

They beat me with a weapon and took me to another room, then they beat my family and poured acid on them, Sultan Mohammad, the father of the family, told Reuters on Friday.

He said the men had split open his head when they attacked him with their guns, and then tied his hands so he could not stop the assault on his wife and daughters.

Acid is used intermittently as a weapon in Afghanistan, but not always against women. In the conservative Taliban-influenced south and east, it has been thrown at girls attending schools.

With foreign combat troops set to return home by the end of 2014, some activists inside and outside Afghanistan fear that women's rights may be sacrificed in the scramble to ensure the West leaves behind a relatively stable state.

Acid attacks have also targeted men. In January, veteran Afghan journalist Abdul Razaq Mamon, a presenter, commentator and author, was left with burns to his hands and face after acid was thrown at him in Kabul.

(Editing by Jan Harvey and Ron Popeski)