An armed gang of kidnappers has abducted three tourists and killed a fourth in the city of Timbuktu in northern Mali, security sources said.
Two of the hostages are Dutch and the third a South African who may have lived in the UK, reports say. The nationalities have not been confirmed.
The dead man, said to be German, was shot dead trying to resist the gang.
It is believed to be the first time foreigners have been abducted in Timbuktu, once popular with tourists.
However, a group linked to al-Qaeda has attacked Westerners in nearby regions.
Following several kidnappings, the UK has warned its citizens not to travel to northern Mali, including Timbuktu.
On Thursday, two French geologists were kidnapped by an armed gang in the eastern village of Hombori.
The Timbuktu gunmen burst in as the four were dining in a restaurant on the central square of the ancient city.
They ordered the tourists there to follow them, a customer at Amanar restaurant told the Associated Press.
The owner of a hotel, where the four have previously stayed during their travel around Mali, told the BBC News website that one of them, a German, had been shot dead when he refused to get into the attackers' vehicle.
He said he had been told by colleagues in Timbuktu that all foreigners in the city had been gathered at the police headquarters and would be flown to the capital, Bamako, on Saturday.
The incidents are the latest in a series of abductions of foreigners believed to be the work of Al-Qaeda in the Islamic Maghreb (Aqim).
Correspondents say Aqim has bases in the northern Mali desert from which it organises raids and kidnappings, and traffics weapons and drugs.
French soldiers have joined Mali's army in the hunt for the French pair kidnapped in Hombori, according to AFP.
The pair were the first Westerners in Mali to be kidnapped south of the River Niger.
The captives were named as Philippe Verdon and Serge Lazarevic, in documents seen by AFP.
They had been sent by the company Mande Construction Immobiliere to take soil samples in the Hombori region where it planned to build a cement factory.
A security guard at their hotel said that "the kidnappers were armed to the teeth".
"I was tied up and told to point out the rooms of the Frenchmen, whom they brutally took away," he said.
Al-Qaeda-linked fighters have in the past brought hostages into northern Mali from neighbouring countries, such as Niger where four French nationals - still being held - were kidnapped in September 2010.
Huge swathes of the Sahel region south of the Sahara Desert have been rendered off-limits to foreigners.
Timbuktu is one of the many former tourist destinations in Mali that foreign embassies have deemed too dangerous for visitors due to the risk of kidnapping by Aqim.
The UK Foreign Office website says there is a "high threat from terrorism" in Mali.
"Terrorists have been involved in kidnaps in the region, on a number of occasions leading to the murder of the hostages," it said.
"We believe that further kidnap attempts are likely."