The Swiss city of Geneva raised its alert level on Thursday and said it was looking for suspects who, according to national officials, had possible links to terrorism.
A security guard at the United Nations' European headquarters told Reuters that Swiss authorities were searching for four men believed to be in or near the city.
Another guard said the U.N. compound was on maximum alert, and Geneva prosecutors said they were investigating the preparation of criminal acts.
Separately, the Swiss attorney-general said it opened an a criminal inquiry on the basis of a "terrorist threat in Geneva" against unknown persons suspected of belonging to a criminal organisation and of violating the ban on al-Qaeda or Islamic State operating in the country.
The Geneva daily Le Temps reported that a friend of Salah Abdeslam, the latter wanted in connection with the deadly Paris attacks on Nov. 13, was in a van spotted by Geneva police on Tuesday after a tip from French authorities that the two men in the car were strongly suspected of ties to radical Islam.
The van, which had Belgian plates, crossed the border into France, the paper said. Geneva officials could not confirm the report.
A French police source said Swiss authorities had been in touch to ask for information, about some suspects, including photographs.
Swiss federal police in the capital Berne said they had passed on information about people with possible links to terrorism, but were not connecting them to Islamist militant attacks in Paris last month in which 130 people were killed.
Earlier, the newspaper Le Matin said a Belgian-registered car that drove through a police check prompted police to examine a photograph of four suspected Islamist militants provided by the U.S. Central Intelligence Agency. The paper said it had obtained a document describing the men as "armed and dangerous".
Two sources confirmed that the CIA had provided the photo, which shows the four bearded men seated, with their faces blurred but index fingers raised in the air. A CIA spokesman in Washington declined to comment.
Swiss television said the city's Jewish community had been told to be vigilant.
"Sensitive sites have been alerted," a Swiss official said.
The guards stationed at vehicle entry points to the U.N. grounds were, unusually, carrying Mp5 sub-machine guns on Thursday. One guard said the U.N. premises had been evacuated for a time late on Wednesday night "as a precaution".
The sprawling complex sits at the heart of "international Geneva". The headquarters of the World Health Organization, the U.N. human rights office, the refugee agency UNHCR, the World Trade Organization and the International Committee of the Red Cross are a short walk away.
"The heightened security affects the entire Geneva area, and the U.N. is taking measures that are commensurate with those taken in the host country," U.N. spokesman Rheal LeBlanc said.
Senior U.S. and Russian diplomats are set to hold talks on Syria in Geneva on Friday, but the United Nations said the location would be kept secret.
Swiss and French officials say they have been working closely together since the Paris attacks. The Swiss Attorney General's office is currently conducting 33 criminal proceedings linked to Islamist militancy, and opened nearly a dozen new investigations in October and November, a spokeswoman said.