The Foreign Ministry says it is trying to locate some 700 Swiss citizens known to be in the United States. Some 3,000 people have called a special hotline set up by the ministry to provide information to anxious friends and relatives.
Foreign ministry spokesman, Walter Thurnherr, told swissinfo that it had identified about 700 Swiss who were in the US, and that it was trying to find out which of them may have been affected by the terrorist attacks.
"We're trying to establish contacts with the authorities in New York and with the hospitals to find out who might be involved and exactly what happened."
He added that it was likely to be "at least ten days, if not two weeks, until we have a clear picture of Swiss victims".
The ministry said earlier that it knew of one Swiss citizen who had been on the passenger list of one of the aircraft, which was hijacked and used to destroy the World Trade Center in New York. It added that it had no way of knowing whether the person concerned - a woman - had actually taken the flight.
Two employees of the Swiss bank, UBS, were also reported missing following the attacks in New York. "We don't know where they are. We are continuing to make inquiries," said a bank spokeswoman, Monika Dunant.
Credit Suisse First Boston, based in New York, said it has undertaken a search for missing employees. Some 800 employees were evacuated on Tuesday from an annex building at the World Trade Center.
Remembering the victims
In Switzerland, churches across the country on Wednesday rang their bells at 1pm in a gesture of solidarity for the victims of the attacks. Bus and tram services in Bern were suspended to allow drivers to observe a minute's silence, along with members of the public.
Some 1,300 people attended a special ecumenical service at Bern's city cathedral, and services also took place in other Swiss cities. The foreign minister, Joseph Deiss, attended the service in Bern.
The Swiss president, Moritz Leuenberger, called the American ambassador, Mercer Reynolds, to express his sadness and disgust at what has happened in the United States. For his part, Reynolds thanked the Swiss people for their sympathy.
Swiss flags flew at half-mast for the victims of the attacks.
Federal authorities have stepped up security around the embassies of the United States and Israel in the Swiss capital, Bern. Police have erected a barricade at the US embassy in Bern, but were allowing residents through to lay flowers outside the building.
The Defence Ministry said police and special forces would be on duty, but no regular army troops would be involved.
The national airline, Swissair, has suspended its regular flights to the US and Israel for security reasons until further notice.