Police and coastguards are searching for a 14-year-old girl who was last seen "in difficulty" in the sea.
Officers were called at 3.46pm to a report of concern for the girl, who was at Whitmore Bay, Barry Island, in South Wales.
The girl is described as white, 5ft 7in tall, of average build with long black hair and last seen wearing a multi-coloured dress.
A force spokesman said: "South Wales Police together with HM coastguard are currently searching the location and a missing person enquiry is ongoing."
Inspector Bob Nicholls of South Wales Police said the teenager was in the water with friends when she got into difficulty.
"She was in the water with friends," he said. "We believe she has got into some difficulty and we are looking for her at the moment.
"The coastguard has got three boats launched, South Wales Police helicopter and the search and rescue helicopter have been deployed.
"Numerous resources on foot have been deployed for the search.
"She was on holiday in the area. Her parents are aware and with officers at the moment."
Coastguards believe the girl could have been in difficulty in the water for up to an hour before the situation was reported to them.
Steve Jones, rescue centre manager for HM Coastguard Swansea said: "The incident was reported to us at 15.54 but we believe she could have been in trouble in the water up to an hour before then.
"We had a report that a fisherman saw her.
"We are dumbfounded that something wasn't said to us earlier and are investigating but our main priority is the search, which is massive.
"We have the HM Coastguard helicopter up from Portland, we have the South Wales Police helicopter and three RNLI lifeboats on the scene.
"A coastguard rescue team from Barry is continuing to search landside.
"We don't believe that she was that far out in the water. We are going on the assumption that she has not come out of the water.
"Normally, if she had come out then we would have found her by now.
"The police are checking the address where she was staying. They are speaking to her friends who were in the water with her.
"We are really concerned and ask that if anyone has any information that they contact us.
"If she is still in the water, as the time passes by, our concerns grow even deeper."
Mr Jones said the team would continue searching for as long as possible, though the helicopters would have to refuel at some stage.
"The girl has to be somewhere," he said. "We just hope she is on land but are concentrating our efforts seawards."
Plaid Cymru councillor Steffan Wiliam, who lives on Barry Island and represents the area for the Vale of Glamorgan Council, said he was "very concerned" to hear the news.
Mr Wiliam said: "Our stretch of coast has the world's second highest tidal range, second only to the Bay of Fundy.
"The main issue is that Whitmore Bay is a pocket shape. You have to be careful but it is generally very safe, particularly now we have lifeguards.
"If you swim between the two strips of flags you should be fine. You always have to be sensible on the beaches here.
"I am very concerned to hear that this girl has gone missing. I am the parent of three kids myself and it is awful to hear news like this."
A spokeswoman for the RNLI confirmed that volunteers are still looking for the teenage girl.
She said: "Three of our lifeboats are still searching. We have one all-weather from Barry Dock and two inshore boats from Penarth."
Alun Cairns, MP for the Vale of Glamorgan, tweeted: "I am very concerned about the 14yr old girl who has gone missing off the Barry Island coast."
Mr Cairns said he had offered his help and support to those searching for the missing girl.
Mr Cairns said: "The RNLI operation is exceptionally efficient at Barry.
"It has all of the latest equipment with significant investment there in recent years from the search equipment to the skills and experience of the operators.
"The coastguards are obviously doing all possible and the coordination between all concerned is operating as it should in the hope that we can find the missing girl.
"There are some concerns that this wasn't brought to their attention immediately but it is important that we focus our search before analysing other things."
Mr Cairns said the stretch of coast where the incident occurred is popular with locals and tourists.
"In spite of Andy Murray playing at Wimbledon, I know that the beach was relatively busy at the time," he said.
"Whitmore Bay is an enclosed area although the tidal movement is significant. Currents can be difficult but that is usually further out.
"This kind of incident, thankfully, does not happen very often here.
"I know Barry intimately, I live a few miles away and it is a popular attraction. For many it is the focus of Barry, which is Wales's largest town."
A spokeswoman for the Maritime & Coastguard Agency confirmed that the girl had been seen in difficulty an hour before the alarm was raised.
She said: "At 3.54pm Swansea Coastguard was contacted by South Wales Police reporting that a 14-year-old girl was missing and was last seen struggling in the water an hour earlier at Whitmore Bay near Barry.
"Swansea Coastguard requested an air and sea search involving the Coastguard rescue helicopter from Portland, Barry Coastguard Rescue Team, Barry Dock and Penarth RNLI lifeboats, RNLI beach lifeguards and officers from South Wales Police.
"After an extensive search extending outwards from the last reported sighting, the girl has not been found."
Steve Jones, rescue coordination centre manager at Swansea Coastguard, added: "If you see someone in difficulty in the water you should call 999 and ask for the Coastguard immediately as any delay means vital search and rescue time is lost.
"Occasionally the Coastguard receives false alarms when somebody who appears to be in difficulty does not need help but we would rather you call 999 anyway if you are genuinely concerned about a situation".
Anyone with information is asked to call South Wales Police on 101.