Police in Geneva raised the alert level and searched the city on Thursday for several suspected jihadists believed to have links to the Islamic State (IS) group, security sources said.
A statement from the Canton of Geneva`s security department said it had received information on Wednesday from Swiss federal authorities concerning "suspicious individuals likely to be in Geneva or the Geneva area."
Police were "actively looking" for these individuals "in the context of the investigation following the Paris attacks", the statement said, while reinforcements were deployed at key locations including United Nations buildings.
"We went from a vague threat to a specific threat," Geneva security spokeswoman Emmanuelle Lo Verso told Swiss Radio, adding that the search for the suspects was at "a very active phase."
A security source at the United Nations complex in Geneva told AFP that the search was for four people with possible ties to IS, which claimed the attacks that killed 130 people in Paris last month.
The individuals were not believed to have any direct link to the Paris atrocities.
The UN source said the Palais des Nations -- the UN`s European headquarters -- was evacuated on Wednesday night as security personnel conducted office-to-office searches.
Security guards posted at the UN gates were also carrying sub-machine guns on Thursday, a departure from normal practice.
The statement from Geneva`s government said police "have increased their level of vigilance", while the ATS new agency reported that security reinforcements were being deployed at key buildings around the city, including the headquarters of major international organisations and the airport.
According to ATS, the heightened alert level has not led to the cancellation of a major festival due to be held in Geneva`s historic old city over the weekend.
Geneva is just over 500 kilometres (310 miles) from Paris and the Swiss city is almost entirely enclosed by France, with border crossings often unmanned.
Police in the two French regions bordering Geneva -- Ain and Haute-Savoie -- told AFP that controls had been beefed up at all crossings.
"The police, the gendarmes and the customs officers are on the ground and are reinforcing their inspections," said a police spokesman in Ain, with similar comments from an official in Haute-Savoie.
Last month, French police searched the homes of two imams from Geneva`s main mosque, which is just metres from the UN complex.
The imams reportedly live in the French border town of Ferney-Voltaire.
Swiss media reports said leaders at the mosque had been preaching extremist ideology, but there was no indication from police that the two imams were implicated in any wrongdoing.
The searches in Geneva, though apparently not directly linked to the Paris attacks, come amid widening investigations across Europe into possible IS sympathisers.
Switzerland said last month that it had active criminal proceedings against 33 individuals over suspected ties to extremist Islamist groups, but only three people were in custody.
The most serious case involves a cell of possible Islamist radicals uncovered in the Canton of Schaffhausen, near the German border, last year.