Indian police Friday arrested and questioned 33 people aboard a ship operated by a U.S. anti-piracy firm for carrying guns and ammunition in Indian waters without proper permits, reports said.

India's coastguard stopped and detained the ship off the Indian coast on October 12 after discovering the cache of weapons and ammunition, before escorting it to the southern port of Tuticorin.

Police then launched an investigation into the 10 crew and 25 security guards of the Seaman Guard Ohio which is registered in Sierra Leone and belongs to the U.S.-based maritime security firm AdvanFort.

The 35, who include British, Estonian, Ukrainian and Indian nationals, were detained on Friday over the stash of some 35 assault rifles and around 5,600 rounds of ammunition, according to the Press Trust of India (PTI) news agency.

Thirty three of them were taken to a local police station for questioning, while two remained on board, PTI said, citing police sources.

The crew had not maintained a log of the arms and did not have the proper permits to carry them in Indian waters, according to NDTV, also using unnamed sources.

The incident comes after a furore erupted in India last year over the shooting deaths of two Indian fishermen allegedly by two Italian marines off the coast of the southwestern Indian state of Kerala.

The southern tip of India is close to major trading routes from Asia and Europe and some cargo ships now travel with armed guards to deter pirates.

The British High Commission or embassy in New Delhi said its officers were attempting to provide assistance to the six British nationals on board and to clarify why they were being detained.

"We are still waiting to receive permission from the authorities to get access to them," a spokesman in New Delhi told AFP.

"We are also trying to better understand what the situation is, to understand why they are being investigated," the spokesman said.

AdvanFort said its staff provided "armed counter-piracy protection" to cargo ships and therefore their weapons and ammunition were stowed on board the Seaman Guard Ohio.

"As these men routinely provide armed counter piracy protection, they also had aboard their uniforms, protective equipment, medical kits, rifles and ammunition - all of which is properly registered and licensed to AdvanFort," the company said in a statement.

AdvanFort also played down the ship's seizure, and instead thanked the Indian coastguard and police for allowing the ship to refuel and escape a cyclone that hit India's eastern coast on the weekend.

"I want to personally thank the Indian government for offering a safe harbor during this typhoon to the crew of our good vessel OHIO,? the firm's president William Watson said in the statement this week.

"We look forward to returning this vessel to its duties as quickly as formalities and resupply operations are concluded."

However Indian Shipping Minister G K Vasan said on Friday that the ship would not be allowed to leave until the investigation was over.

"We are still to find answers to many questions, the probe will bring out the truth. Till then, the ship will not be allowed to move out of Tuticorin port," Vasan told reporters.

Deputy National Security Advisor Nehchal Sandhu said on Thursday that the investigation would be dropped if it was determined the ship had been outside of Indian waters.

"You have to understand that our territorial waters extend up to 12 nautical miles," Sandhu was quoted by PTI as saying.

"Anything that happens beyond that is not within the realm of our control."