A former backup performer for R. Kelly testified that she saw Mr Kelly engaging in a sex act with the R&B singer Aaliyah around 1993 when Aaliyah was only 13 or 14 years old.

The woman, who testified under only the name Angela, told jurors at Mr Kelly’s criminal trial in Brooklyn that she was performing with Mr Kelly on tour when she went to visit the singer on his tour bus.

But as she began to open the door, Angela said, she saw Mr Kelly, who was in his mid-20s at the time, and Aaliyah “in a sexual situation,” she told jurors. Aaliyah was seated in a chair, she said, and Mr Kelly was kneeling and appeared to be performing a sex act on the girl.

Angela “closed the door abruptly,” she said, and never spoke with Mr Kelly about what she saw.

The testimony came on the 15th day of Mr Kelly’s trial, during which six accusers, including Angela, have said that Mr Kelly had sex with them while they were underage.

But Angela’s account stood apart from others: Aaliyah, whom Mr Kelly married illegally in 1994, is the youngest girl that the singer has been accused of sexually abusing. And Angela became the first accuser to testify that she had seen the two engaged in sexual acts. Aaliyah, whose full name was Aaliyah Dana Haughton, died in a plane crash in 2001.

Mr. Kelly, 54, faces one count of racketeering and eight counts of violating the Mann Act, which prohibits transporting people across state lines for the purpose of prostitution. He has denied the accusations and pleaded not guilty to the charges against him.

Also on Monday, a second male accuser testified against Mr Kelly. The witness, identified only as Alex, said Mr. Kelly groomed him for sex beginning when he was 16. He said that they began having sex when he was 20, and said that from 2007 to 2019 he was “brainwashed” by the singer.

Aaliyah, who was one of the most celebrated music stars of the 1990s, has been a significant focus of Mr. Kelly’s trial. The marriage of the two singers, using falsified documents when Aaliyah was 15 and Mr. Kelly was 27, prompted the first significant scrutiny of his encounters with underage girls.

Potential sexual crimes that date to the early 1990s would typically be too old to prosecute. But the racketeering charge, which claims that the singer, whose full name is Robert Sylvester Kelly, was the head of a criminal scheme that preyed on women and underage girls for sex, allows the government wider latitude.

Angela said she first met Mr Kelly around 1991 when she was between 14 and 15 years old. The singer began to have sex with her while she was underage and a student in high school, she testified.

Angela recalled that Mr. Kelly often pressured her and several others who worked around him as backup performers to have sex.

She told jurors that she first met Aaliyah shortly before the young singer began to work on her first album, “Age Ain’t Nothing but a Number,” with Mr. Kelly. Aaliyah travelled on a tour with him to “get her feet wet” in the industry around 1992 or 1993, Angela told jurors, but it was during that trip, she said, that she saw the two engaged in a sexual act.

Aaliyah would have been 13 or 14 at the time.

On a separate stop in Washington, D.C., during the tour, Angela recalled that she and a group of other girls including Aaliyah were reprimanded for breaking the singer’s instructions. Angela told jurors that they had made a trip to a 7-Eleven convenience store for food, but that Mr. Kelly had told them not to leave their hotel. When he learned of their disobedience, he demanded sex, she said.

“Robert told all of us we would have to put out,” she said. “It was dues time,” she added, explaining to jurors that the singer often presented having sex with him as a form of paying their dues.

Angela said that she refused the singer’s advances. While Aaliyah was among the girls that Mr. Kelly had addressed, she was not aware whether the two had sexual encounters immediately following that incident.

Alex, the male accuser who appeared on Monday, also testified to “countless” sexual encounters with the R&B superstar and other women at Mr. Kelly’s direction.

He testified that for more than a decade he often saw the singer at parties and pickup basketball games — what he called “Kelly ball,” in which “you’d get the ball, rebound, let him shoot” — at a recreational centre and a church gymnasium in Chicago.

Mr. Kelly first made an advance on him inside the singer’s Maybach, when Alex was about 20 years old, he said. “I recall him just talking to me, then forcefully kissing me, licking my face,” he said, adding that afterwards, Mr. Kelly told him “just to be open-minded.”

During sexual encounters with Mr. Kelly, he said the singer referred to him as “nephew,” although, Alex testified, they are not related. He said he was directed to call Mr. Kelly “Daddy.” Sometimes afterwards, he said, Mr Kelly would “slap $100 or two in my hand just for gas, my time.”

On one occasion, he said Mr. Kelly told him he had a gift for him, and instructed him to wait in a room Alex said he believed was the bedroom of one of the singer’s children. There, Mr. Kelly had set up an iPad and production lights and directed Alex to take off his clothes. Then, the singer “sent the half-naked lady into the room.”

“He pretty much told us to go at it, but we didn’t know what to do because he was still in the room,” he said, adding that the singer then “told us to start kissing each other — and to mean it.”

Mr. Kelly began recording, Alex said, and then masturbating as he directed them through a series of sex acts.

During the encounters, the women — whose names he did not know — appeared “zombie-ish,” he said.

But it was Mr. Kelly’s interactions with Aaliyah that began to focus public scrutiny on accusations of sexual misconduct by the singer.

In the mid-1990s, Vibe magazine reported the details of the fake identification that was procured so Aaliyah could marry Mr. Kelly in 1994. Witnesses have said that the union came about because Mr. Kelly believed that Aaliyah may have been pregnant and feared that he could be prosecuted for statutory rape.

And as Mr. Kelly’s conduct came under renewed scrutiny at the height of the Me Too movement, both instalments of the “Surviving R. Kelly” documentary series delved into the accusations.

“If people would have protected Aaliyah, so many other girls wouldn’t have gotten touched,” a former boyfriend, Damon Dash, said during the second portion of the Lifetime series. “Aaliyah was like the sacrificial lamb for all that.”

Several witnesses during the trial have testified about other aspects of the encounters between Mr. Kelly and Aaliyah. The minister who married the two spoke publicly for the first time earlier this month, telling jurors that he was asked to give his word that he would not discuss their 10-minute wedding ceremony at a Chicago-area hotel.

And one of the singer’s former tour managers, Demetrius Smith, testified that he bribed a government employee for the fake identification for Aaliyah as Mr. Kelly’s inner circle began to plot the marriage.

The union was annulled in 1995, and Aaliyah filed to have details of their union expunged from her record two years later. Her subsequent albums were produced without Mr Kelly.

During his testimony, Mr. Smith described the first meeting of the two singers in 1992. He said they began working on her music together and appeared to be “more than friends from the very beginning,” though he said he first viewed their interactions as a uniquely close working relationship.

But over time, the friendliness of their exchanges began to raise concerns, Mr. Smith told jurors.

“At times, I was concerned,” he testified. On at least one occasion, Mr. Smith said, their behaviour had prompted him to ask, “Robert, you ain’t messing with Aaliyah?”

Mr. Smith said he meant “flirting, being flirtatious. Seducing her.”