By Ruma Paul

DHAKA (Reuters) - Police in Bangladesh were hunting on Thursday for two of the three suspected Islamists who attacked and seriously injured a Hindu college teacher in the latest assault on minority groups and liberal activists in the mainly Muslim nation.

Bangladesh has arrested more than 11,000 people in a week-long crackdown on Islamists begun on Friday, as Prime Minister Sheikh Hasina has vowed to halt a wave of targeted killings.

Mathematics teacher Ripon Chakraborty was attacked on Wednesday night by three knife-wielding assailants when he answered the doorbell at his home in Madaripur, 70 km (44 miles) south of the capital, Dhaka, police said.

"The 50-year-old teacher was hacked in his head and neck," said district police chief Sarwar Hossain. "He is now out of danger and doctors have kept him under close observation."

Police are questioning one of the attackers, a 22-year-old student of a college in Dhaka, Ghulam Faijullaha Fahim, who was caught by people who came to the rescue of the screaming teacher and his family, he added.

Under interrogation Fahim revealed the attack had been motivated by members of a banned militant group, the Hizb ut-Tahrir, Hossain said, adding, "He was told, 'This person is an enemy of Islam. He should be killed.'"

Militants have killed more than 30 people since early last year, ranging from atheist bloggers and liberal academics to gay rights campaigners, foreign aid workers, members of minority Muslim sects and other religious groups.

Islamic State or al Qaeda have claimed responsibility for most of the killings, but the government denies either group has a presence in Bangladesh. Police blame home-grown militants from groups such as Jamaat-ul-Mujahideen and Ansarullah Bangla Team.

In the past week alone, an elderly Hindu priest, a Hindu monastery worker and a Christian shopkeeper were hacked to death, and the Muslim wife of a counter-terrorism police official was also killed.

Hindus constitute about 9 percent of Bangladesh's population of 160 million.

(Reporting by Ruma Paul; Editing by Clarence Fernandez)