Bahraini authorities have conducted a systematic campaign of attacks and arrests against medical workers who treated injured protesters during months of unrest in the Gulf kingdom, according to a US-based medical group.

Physicians for Human Rights claims doctors and nurses have been targeted because they have "evidence of atrocities committed by the authorities, security forces and riot police" in the crackdown on Shia dissent in the Sunni-ruled island nation, which has been placed under martial law.

The report by the group is the second to accuse Bahrain's authorities of abuses in the medical system. Earlier this month, the international humanitarian organisation Doctors Without Borders called Bahrain's hospitals "places to be feared" and claimed officials were arresting protesters who sought treatment.

The military took over the main Salmaniya medical centre in Manama last month, and doctors and patients said soldiers and police had conducted interrogations and detentions inside the complex.

Physicians for Human Rights said at least 32 healthcare professionals had been detained since Bahrain declared martial law last month to quell anti-government demonstrations by the country's Shia majority demanding greater freedoms and equal rights.

The report details attacks on physicians, medical staff and patients "with weapons, beatings and teargas".

"These attacks violate the principle of medical neutrality and are grave breaches of international law," it says, adding that "high-velocity weapons and shotguns" have been used to fire birdshot, rubber bullets and teargas at unarmed civilians.

About 1,500 troops from Saudi Arabia and the other Gulf states are in Bahrain to help the ruling monarchy. Sunni leaders around the Gulf fear gains by Bahrain's Shias could open the way for greater influence by Shi'ite powerhouse Iran in the strategic nation, which is home to the US navy's 5th Fleet.

At least 30 people have died and hundreds of demonstrators, opposition leaders, activists and Shia professionals such as doctors and lawyers have been detained since anti-government protests started 14 February.