The Swiss honorary consul to Liberia, Jürg Landolt, has gone missing in the country's capital, Monrovia.
Family friends say he was kidnapped by rebels staging an attempted coup.
The Swiss foreign ministry said Landolt disappeared in Monrovia on Saturday, along with his assistant.
They went missing as the rebel group Liberians United for Reconciliation and Democracy (LURD), surged towards the capital, in a bid to overthrow the president, Charles Taylor.
Muriel Berset-Kohen, a spokeswoman for the foreign ministry, said the fate of the missing party was not yet clear.
"We are in contact with the European Union mission in Monrovia and our embassy in Accra, Ghana," she told swissinfo. " But we cannot confirm that they have been kidnapped."
However, a friend of Landolt's family told the Associated Press news agency that Landolt, his assistant and two other Europeans, had been taken by the rebels as they temporarily retreated from the capital.
Government troops were trying to prevent the rebels from storming the city on Sunday, while a senior LURD member reportedly gave Taylor a 72-hour ultimatum to step down.
Thousands of civilians have fled Monrovia for safety, but many found escape routes blocked by rebels, who hold two-thirds of the country.
"Conditions are dreadful," said Magnus Wolfe-Murray of British aid agency Merlin.
"There are anything between 300,000 and 700,000 people in Monrovia without anywhere to stay," he added.
There are two rebel factions in Liberia trying to depose Taylor, amidst violence that has rocked the country for nearly 14 years.
Taylor has been indicted by a United Nations-backed court for war crimes in Sierre Leone, and the United States has told him to step down and face the court.
swissinfo with agencies
Swiss honorary consul, Jürg Landolt, went missing with his assistant in the Liberian capital, Monrovia, on Saturday.
His disappearance coincides with an attempt by rebels to overthrow the Liberian president, Charles Taylor.
The Swiss Foreign Ministry says there is no confirmation Landolt was kidnapped, but a family friend says he was taken by rebels.