A Peruvian prosecutor has said that two women from Britain and Ireland caught trying to smuggle cocaine out of the country would need to make fuller confessions if they hoped to have their sentences reduced.

Irish national Michaella McCollum, 20, and Melissa Reid, 19, pleaded guilty on Tuesday to attempting to smuggle 11 kilograms of cocaine out of Peru.

They had initially denied guilt, claiming they had been coerced into smuggling by a shady international cartel after being kidnapped.

However they later changed their story stating they "knew they were going to transport drugs and that they regret having participated in such an act," according to a court press release.

Prosecutor Juan Rosas told AFP Wednesday the pair would need to give a fuller confession if they wanted to see their likely prison sentences slashed from eight years to six years and eight months.

"Prosecutors will not be shortening the length of their sentence necessarily because the acceptance of responsibility really is quite limited," Rosas said.

"This is not a door being closed once and for all," Rosas stressed, adding: "We have asked the judge for another step so that the suspects can expand on their statement, and give more details about the facts under investigation, and fully acknowledge their guilt."

Both were arrested on August 6 at the international airport in Lima, when 5.7 kilos (12.5 pounds) of cocaine were found in McCollum's luggage and 5.8 kilos in Reid's. They were trying to board a plane for Spain where they had been working on the party island of Ibiza.

After their arrest, the women told British reporters that they had been kidnapped by a drug cartel, taken to Peru and forced to transport drugs.