The humanitarian aid group Medecins Sans Frontieres has announced it is suspending operations in Russia's North Caucasus region, following the abduction of a Dutch aid worker earlier this week.

Peter-Arjan Erkel was kidnapped when three gunmen pushed him into a car in the Dagestani capital, Makhachkala, late on Monday evening.

The organisation, which supplies $6m worth of medicines to the region every year, has already suspended operations in neighbouring Chechnya.

So far no group has come forward to claim responsibility for the abduction.

Ransom demands

"MSF is extremely concerned about Arjan's fate and urges whoever is holding him to respect his physical and mental integrity and to release him unharmed," the aid organisation said in a statement.

Arjan Erkel was bundled into a car by gunmen

Kidnapping has become commonplace in many of the Caucasus republics.

Most of the abductions are thought to be carried out by Chechen separatists who demand high ransoms to fund their war for independence from Russia.

But no ransom demands have yet been received for 32-year-old Mr Enkel, who had been working as the head of the Swiss section of MSF since April.

In the past, most of the people taken hostage were Russians, but in recent months foreigners working for charities and non-governmental organisations have also become kidnap targets.

MSF suspended work in Chechnya in July after the abduction of Nina Davidovich, the head of a local aid agency. The United Nations also suspended its operations there at a similar time.

Aid workers are needed in the region to provide medical help to tent camps across Dagestan and neighbouring Ingushetia, where some 150,000 refugees are sheltering from the Chechen war.

MSF also funds mobile clinics, hospitals and drug distribution programmes in Chechnya.

The organisation's statement acknowledged that the situation was precarious and the medical needs still great.

It said it would "continuously review the situation to decide whether to maintain the suspension".