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 department said in a statement, while reinforcements were deployed at key locations including United Nations buildings.
In a statement, Geneva justice authorities said the public prosecutor's office had opened a criminal investigation into "unlawful preparatory acts" that is being led by cantonal attorney general Olivier Jornot and chief prosecutor Yves Bertossa.
The investigation was launched after Swiss officials received information from the US about suspects actively being sought, the Tribune de Genève newspaper reported online.
"We went from a vague threat to a specific threat," Geneva security spokesman 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 the Islamic State group which claimed the attacks that killed 130 people in Paris last month.
The Tribune de Geneve reported that an additional two people were being sought.
The individuals were not believed to have any direct link to the Paris atrocities.
UN offices evacuated
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 organizations and the airport.
The development emerges a day before representatives of the US, Russia and the United Nations were scheduled to hold talks on the Syria crisis in Geneva.
Russian deputy foreign minister Gennady Gatilov said Russia will present its "vision" for intensifying joint efforts to fight terrorism in Syria.
U.S. State Department spokesman John Kirby said Assistant Secretary of State Anne Patterson, the top US diplomat for the Middle East, will represent Washington at the meeting.
Kirby was quoted as saying the talks would "try to get at the framework and the architecture for a cease-fire".
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According to ATS, the heightened alert level has not led to the cancellation of L'Escalade, a major festival due to be held in Geneva's historic old town on Sunday.
Organizers said that extra police patrols would be instated for the event, which features 800 people marching in historic costumes to celebrate a historic victory in 1602, when Genevans repelled attacking soldiers from Savoy.
Geneva is just over 500 kilometres from Paris and the Swiss city is almost entirely enclosed by France, with some border crossings often unstaffed.
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.
Guards armed with machine guns outside the entrance to UN headquarters in Geneva. Photo: Richard Juilliart/AFP