Detectives will continue to investigate the rock singer Ian Watkins after he admitted a string of child abuse offences including the attempted rape of a baby.

Watkins, the 36-year-old lead singer and founding member of the now disbanded Lostprophets, faces a lengthy jail sentence, but officers fear he may have used his celebrity status to exploit more as yet unknown victims both in the UK and overseas.

Detectives in south Wales, where Watkins lived, have been working with other forces and agencies across Britain and have liaised with Interpol and the department for homeland security in the US, where Lostprophets toured extensively.

Watkins and two female fans in their 20s, who cannot be named, had been due to stand trial at Cardiff crown court for more than 20 offences, including allegations involving the women's children, a boy and a girl. He had vociferously denied the accusations and loyal fans had attended court to show their support.

But on Tuesday – after jurors had been warned they would have to examine some very disturbing images and arrangements had been made for them to receive counselling after the trial – Watkins and the women pleaded guilty to almost all the charges against them.

Watkins, whose former band has sold around 3.5m albums, admitted 13 offences over five years, including attempting to rape one of the children and conspiring to rape the other.

The court was told that the two women sexually abused their children at the behest of Watkins and were prepared to make the children available to him for sex. The prosecution said the attempted rape happened while Watkins was staying at a four-star hotel in west London shortly after appearing on BBC Radio 1.

When police investigated Watkins, who is from Pontypridd, they found a secret disc. GCHQ experts helped them get access to the material on the disc, which featured videos of sexual abuse, including the attempted rape.

Watkins, a user of crack cocaine and crystal meth, maintained he could not remember the incident but the court was told this could have been because he was high on drugs. It emerged he had also discussed forcing his victims to take drugs – and one of them was found to have been exposed to crystal meth.

Christopher Clee QC, prosecuting, branded Watkins a "determined and committed paedophile". The court heard of one exchange in which a woman offered him a "summer of child porn". He replied: "Hell yes, baby."

Outside court, Catrin Evans, the head of the complex casework unit of the crown prosecution service in Wales, said: "The three defendants in this case have admitted very serious sexual offences committed against extremely young children, demonstrating sexual exploitation at its worst."

South Wales police – who have codenamed their investigation Operation Globe – said detectives had spoken to witnesses around the world and would continue to investigate Watkins in case there were other victims.

DCI Peter Doyle, of South Wales police, said: "This investigation has uncovered the most shocking and harrowing child abuse evidence I've ever seen. There is no doubt in my mind that Ian Watkins exploited his celebrity status in order to abuse young children. Today's outcome does not mark the end of our investigations and we will work tirelessly to identify any other victims."

The South Wales force has referred itself to the police watchdog the IPCC, which is examining if officers reacted quickly enough after four forces passed on information about Watkins before he was finally arrested. The inquiry is ongoing.

Co-founded by Watkins in Pontypridd in 1997, Lostprophets has released five albums. The other members of the band announced they were splitting up last month.

Watkins and the two women will be sentenced next month.