Hertfordshire Constabulary does not know the whereabouts of two convicted sex offenders, according to police records.

The unnamed offenders have failed to provide information about their current addresses, which is a requirement of the Sex Offenders Act.

But the force said "considerable" behind the scenes work has been done to locate them and it is making "positive progress".

Rachel Hyde, spokesman for Hertfordshire Constabulary, said instances of sex offenders going missing are very rare.

She said: "Maximising on all the intelligence and partnership information available, Hertfordshire Constabulary has a successful track record in the management of the county’s registered sex offenders.

"Because of this continued vigilance the instances of sex offenders going missing is very low.

"In the case of these two offenders considerable behind the scenes work has been done to locate them and positive progress is being made.

"Because our inquiries are at a crucial stage we will not be disclosing the identity of the two that have gone missing because that may have a serious impact on the success of our work to apprehend them."

Hertfordshire Constabulary refused to comment how long the two offenders have been missing or what offences they were convicted of – fearing such information would reveal to the pair or their associates the force was closing in.

The figures were released as part of a Freedom of Information request by the Press Association. Responses by 39 forces in the UK show 394 sex offenders are currently wanted because their whereabouts are unknown.

Police forces revealed there were missing registered sex offenders in their areas in February or early March.

Ms Hyde continued: "The force has worked with national and international agencies to successfully locate and manage those few offenders who do go missing and will continue to use all the resources available on an on-going basis."

Registered sex offenders - including rapists and paedophiles - are required to inform police and probation officers of their addresses.

They are supposed to be monitored by officials working under multi-agency public protection arrangements (MAPPA).

The latest MAPPA figures show 628 registered sex offenders in Hertfordshire on March 31, 2014.

The statistics also revealed 27 registered sex offenders in the county had been cautioned or convicted for breach of notification requirements during 2013/14.