Liberia's authorities have quarantined the area where a 17-year-old boy died of Ebola.
This is the first reported case of Ebola in Liberia since it was declared free of the disease seven weeks ago.
Deputy health minister Tolbert Nyenswah said tests confirmed that the teenager from Nedowein village, near the international airport, had died of the disease on 28 June.
Officials are investigating how he contracted Ebola, Mr Nyenswah said.
More than 11,000 people have died of the disease since December 2013, the vast majority of them in Liberia, Guinea and Sierra Leone.
The countries had largely curbed the spread of the disease - but the number of new cases has risen recently, with the start of the rainy season in West Africa.
Analysis: BBC News website health editor Michelle Roberts
The news that a 17-year-old has died of Ebola in Liberia is deeply troubling. The country was thought to be free of the deadly virus - no cases had been reported for the past seven weeks, until now.
Officials are urging people not to panic and instead "go about their business as normal". They say they have the situation in hand - the teenager's body was buried safely and surveillance has been stepped up.
But it is not clear how the young man caught the virus and who he may have been in contact with before he died. Liberians must remain vigilant if they are to banish Ebola for good.
Mr Nyenswah told the BBC's Jonathan Paye-Layleh in Monrovia that the authorities were dealing with the situation effectively and there was no need for the public to panic.
"We have said over and over again that there was a possibility that there could be a resurgence of the virus in Liberia," he said. "But our surveillance team, our capacity is very strong.
"The only complication is that the person died before we tested the body as part of our surveillance system."
The number of people quarantined in Nedowein, about 30 miles (48km) from the capital, would be made available later, Mr Nyenswah said.
The authorities were investigating whether the dead man had contracted the disease as a result of travel, he said.
The man's body had been buried safely, in accordance with guidelines to check the spread of Ebola, he added.