Police in Bangladesh have arrested at least 119 militants as part of a crackdown on Islamists after a wave of deadly attacks on members of minority groups and liberal activists, an officer said on Monday.

More than 8,000 criminal suspects have also been arrested since law enforcement agencies began a week-long drive on Friday to halt the targeted killings in the mainly Muslim nation.

Those 8,000 were accused of firearms, narcotics and other offences, national police chief A.K.M. Shahidul Hoque said. However, Bangladesh's main opposition party says the mass arrests are a ploy to suppress political opponents.

Militants have killed more than 30 people in Bangladesh since early last year, with atheist bloggers, liberal academics, gay rights campaigners, foreign aid workers, members of minority Muslim sects and other religious groups among the victims.

Islamic State has claimed responsibility for 21 of the attacks since its first claim in September last year and al Qaeda has claimed most of the rest, according to Site Intelligence Group, a U.S.-based monitoring service.

The government denies that either group has a presence in Bangladesh and police say home-grown militants from Jamaat-ul-Mujahideen and Ansarullah Bangla Team are responsible.

The groups are seeking to impose strict Islamic law on Bangladesh, whose population of 160 million are mostly moderate Muslims.

Of the 119 militants, most of are members of Jamaat-ul-Mujahideen, said Shahidur Rahman, deputy police inspector general.

Prime Minister Sheikh Hasina has vowed to track down the killers, blaming the growing violence on political opponents linked to Islamist parties.

The main opposition party, the Bangladesh Nationalist Party, led by former Prime Minister Khaleda Zia, said more than 2,100 of its leaders and activists across the country had been arrested under the crackdown since Friday.

Last month, junior foreign minister Shahriar Alam told Reuters that Islamic State was trying to ride a wave of religious radicalisation by falsely claiming killings and said there was enough evidence implicating domestic militant groups.

At least 10 suspected members of the outlawed Jamaat-ul-Mujahideen have been killed in shootouts since November, including five last week, police said.

UN rights chief Zeid Ra'ad Al Hussein said he was concerned about the killings and urged the government to do more to protect the affected groups.-Reuters