US prosecutors on Friday dropped charges against a Saudi princess accused of enslaving a Kenyan woman as a housemaid, forcing her to work in abusive conditions and withholding her passport.

Meshael Alayban, one of six wives of a grandson of Saudi Arabia's King Abdullah, had faced up to 12 years in jail for allegedly making the Kenyan housekeeper work long hours seven days a week for just $220 a month.

The 42-year-old, charged in July, had been due to enter a plea to felony charges of human trafficking. But Orange County District Attorney Tony Rackauckas unexpectedly dismissed the case due to insufficient evidence.

"The allegations were strong and we had reasons to believe they were true," Rackauckas told reporters.

He added: "We were certainly dealing with a family who could leave the jurisdiction in a short time, so I think bringing the case under the circumstances was the right thing to do at the time.

"It did appear the evidence corroborated (the claims), but as we looked deeper into it, it didn't."

Lawyers for the Saudi royal accused the 30-year-old Kenyan, who has not been named, of lying in an attempt to obtain a visa to stay in the United States.

"It was obvious she was seeking (a) visa," said attorney Paul S. Meyer, adding that he suspected the accuser was lying "from the moment this case was filed. It doesn't pass the smell test," he said.

The Kenyan's lawyer said his client was disappointed, but denied that she had lied in an attempt to get a visa. "To say this is about her getting a visa is ridiculous," said attorney Steve Baric.

"We're obviously disappointed," he added.

The Kenyan woman left her home country seeking work to pay for her young daughter's medical care. She reportedly worked in Alayban's palace in Saudi Arabia from last year, and then in her home in Irvine, California, southeast of Los Angeles.

Prosecutors said she had signed a contract with an employment agency that promised her a salary of $1,600 a month for a 40-hour work week.

But the job actually involved working 16 hours a day with no weekends, for a fraction of the promised pay, her lawyers alleged.

The princess was charged with "human trafficking of a Kenyan woman into the United States and forcing the victim to work as a domestic servant against her will."

The victim, who moved to the US with the Saudi family in May, was "forced to work tending to at least eight people in four apartments," prosecutors said.

She was given no breaks, no days off, and no chance to leave "except for a family outing so the victim could carry the family's bags." She told police Alayban withheld her passport and refused to allow her to return to Kenya.

But the Saudi princess's lawyers said the housekeeper traveled to Orange County in first class, had her own cell phone and shopped at neighborhood malls at Alayban's expense.

The alleged victim left Alayban's apartment on July 9 and flagged down a passing bus, claiming that she had been held against her will.

Alayban, a wife of Saudi Prince Abdulrahman bin Nasser bin Abdulaziz al Saud, had been free pending Friday's hearing on $5 million bail reportedly posted by the Saudi consulate, although she was required to wear a GPS tracking device.