Three Swiss nationals have been officially confirmed as missing following the hurricane that struck the city of New Orleans in Louisiana ten days ago.
The Swiss consulate in Houston, Texas, said it was "extremely concerned" for the safety of the two women and one man.
Acting Consul-General Jürg Bono said the US State Department had been notified.
"One woman has not heard from her husband for a week. She was evacuated to Houston, but he stayed to help a neighbour who was stuck in an attic and she has not heard from him since," Bono told swissinfo.
"This case is the one we are most concerned about."
Bono added that the other two listed as missing had been in regular contact with friends or relatives in Switzerland prior to the hurricane.
"Nothing has been heard from them since. This does not mean something has happened to them, but after a week we start to get worried. Communication has been restored in many areas and we would have expected to have heard from them by now."
According to Bono, 92 families totalling 149 people were listed as Swiss or dual nationals living in New Orleans.
He said that immediately after the hurricane the Swiss consulate in Houston emailed 33 families for which it had email addresses, asking them to get in touch. The consulate received 16 replies which provided information about 40 people.
Consulate officials point out that some of the dual-national families would have turned to the US rather than Swiss authorities.
Alabama and Mississippi
Swiss citizens who live in Alabama and Mississippi, the two other states affected by Hurricane Katrina, fall under the jurisdiction of the Swiss consulate in Atlanta.
Staff at the consulate told swissinfo that the vast majority of the 324 Swiss registered as living in the two states were accounted for.
"Most people seem to be in good health and have only minor damages to their homes," said consulate official Monica Camacho-Bandlin.
She added that only "a handful" of the 324 lived in the worst-affected coastal regions. Of these, "about 50 per cent" had been in contact to confirm they were safe.
Camacho-Bandlin made a plea to any Swiss who have not been in touch since the hurricane struck to do so as soon as possible.
Bono said that the past week had been extremely difficult but that staff at the consulate had been relieved by the number of emails received saying people were alive and well.
He said the humanitarian-relief effort was now going well despite "having taken a while to get going".
"The relief effort is working much better than at the beginning. But you have to remember that this was a huge catastrophe. I don't think people expected it to be as big as it was. They were very surprised."
He also praised the reaction of the American people to the tragedy, recalling that 25,000 volunteers had turned up at the Astrodome in Houston to help those who had been made homeless.
Bono added that the American-Swiss Society in Houston had also asked its 120 members to take people in.
swissinfo, Adam Beaumont in Houston, Texas
92 families totalling 149 people were listed as Swiss or dual nationals living in New Orleans, Louisiana.
324 Swiss were registered as living in Alabama and Mississippi, the two other states affected by Hurricane Katrina.
Initial sweeps of New Orleans indicate the death toll may not be as catastrophic as first believed. Earlier estimates were as high as 10,000.
Some 400,000 homes in the city are still without power, with no immediate prospect of getting it back.
Three Swiss nationals have been officially confirmed as missing following Hurricane Katrina.
The vast majority of the 324 Swiss registered as living in Alabama and Mississippi are accounted for.