(CNN) -- Three Red Cross workers kidnapped this week in the southern Philippines have made contact with the aid group to say they are unharmed, but they remain missing, the International Committee of the Red Cross said Saturday.
Mary-Jean Lacaba, Eugenio Vagni and Andreas Notter were seized by men armed with machine guns and riding motorcycles Thursday on the island of Sulu, the ICRC said.
They were in the region to inspect a provincial prison and were en route to the airport when they were kidnapped, the ICRC said.
"We are, of course, relieved to have heard their voices and to know that they are alive," said the head of the ICRC's delegation in Manila, Jean-Daniel Tauxe.
"We are not in a position to comment further, since our main priority is ensuring that nothing jeopardizes their safety, but it goes without saying that their families, as well as their colleagues, are hoping for their safe and swift return."
Philippine security forces are pursuing the abductors, according to Richard Gordon of the Philippine National Red Cross. He has urged the kidnappers to release the workers, saying they are neutral players in the sometimes turbulent southern provinces.
Several Islamic militant groups, some of which have ties to al Qaeda, are active in the southern Philippines. It is unclear who was responsible for the attack.
Last month, armed men attacked a U.N. truck carrying food to the southern Philippines, killing the driver's assistant.
The abduction of the three aid workers has put on hold the ICRC's main activity on Sulu Island, where staff are involved with a water and sanitation project at the jail, the ICRC said.
Sulu is a province of Mindanao, where the ICRC says around 300,000 people are displaced because of fighting between government forces and the Moro Islamic Liberation Front.
The ICRC said its other activities in Mindanao, which include feeding and assisting 120,000 displaced people every month, are unaffected by the kidnappings.