Condoms are still necessary to stop the spread of HIV
Circumcising men who already have HIV does not protect their female partners from the virus, a study in Uganda has found.
Circumcision is known to protect men from acquiring HIV.
But the research, from the Lancet, showed no benefit in those who already had the virus and was stopped early because of the continued risk to women.
Experts say HIV-positive men should still be offered circumcision, but also warned to use condoms.
The US researchers, from Johns Hopkins School of Public Health in Baltimore, say not offering the procedure to men with HIV would stigmatise them.
Other experts say it could become a "sign" of whether a man was HIV positive or not.
Previous research had suggested women could be protected from HIV if their partner was circumcised.
In this study, 922 uncircumcised, HIV-infected, asymptomatic men aged 15-49 years with HIV were enrolled in the Rakai district of Uganda.
Men were then selected to have immediate circumcision (474 men) or to be given circumcision after two years (448 men).
Almost 170 uninfected female partners of the men were also enrolled, and followed up at six, 12, and 24 months.
However, the trial was ended early because of what the researchers called the "futility" of carrying on, and the second group were not circumcised.
Only 92 couples in the immediate circumcision group and 67 in the control group were included in the final analysis.
It was found that a higher proportion of women were infected with HIV in the intervention group (18%) versus the control group (12%).
The researchers suggest the higher transmission rate could have been due to couples resuming their sex lives before the circumcision would have properly healed.
Writing in the Lancet, the team led by Dr Maria Wawer said: "Circumcision of HIV-infected men did not reduce HIV transmission to female partners over 24 months; longer-term effects could not be assessed."
They said it was not sensible to recommend men with HIV should not be circumcised, or that there should be any down-scaling of circumcision programmes, because of the overall benefits to both uninfected men and to women.
But they added: "It's inevitable that men who are infected with HIV will also require to be circumcised, partly to avoid stigmatisation.
"The findings suggest that strict adherence to sexual abstinence during wound healing, and continuous condom use thereafter must be strongly promoted when HIV-infected men receive circumcision."
They suggest a solution would be to circumcise as early as possible.
"Circumcising infants and young boys before their sexual debut would mitigate the challenge of male circumcision in HIV-infected men.
"However, this strategy would require careful consideration of issues relating to parental consent and the minor's consent."
In an editorial in the journal, a team from the University of Washington in Seattle, led by Dr Jared Baeten said the findings should not "hinder" the use of circumcision in HIV prevention programmes.
Victoria Sheard of the UK's Terrence Higgins Trust, said: "Circumcision is most often used as an HIV prevention tool in the developing world, where it can be difficult to get hold of condoms.
"However, it shouldn't be seen as a stand-alone strategy.
"Women are disproportionately affected by HIV in sub-Saharan Africa, and - as this study shows - will still be at risk whether their partners are circumcised or not.
"The best way to guard against HIV is by always using a condom, so more work is needed to ensure adequate protection is available for those who need it the most."