Switzerland's talks with Brussels on curbing immigration from the European Union have been complicated by Britain's vote to leave the bloc as EU officials focus on unravelling that relationship, President Johann Schneider-Ammann said on Friday.

Neutral Switzerland has until February to implement a binding 2014 referendum demanding limits on the influx of foreigners to a country whose population is already a quarter foreign, so needs a deal by summer to have legislation in place.

But with EU preoccupied with the loss of major member Britain, Swiss officials fear, chances are fading for a quick deal with outsider Switzerland.

"The EU signalled a readiness to intensify discussions after the Brexit referendum. At the same time it is clear that the search for a solution has not become simpler with the withdrawal of the UK," he said.

Asked about contacts with EU political leaders, he wryly noted: "This morning, if you're Swiss, you cannot get anything in Brussels."

Talks continue on a technical level, he said, but it was far from clear that a political deal could be struck soon.

Unilateral Swiss curbs remained an option although he would prefer a mutual accord, he told reporters in Bern. Analysts warn that a unilateral move could risk other agreements with the EU.

Switzerland is seeking to negotiate a compromise with Brussels, which has insisted it cannot brook any impediments to the free movement of people enshrined in bilateral accords. Negotiations have been on hold until the Brexit vote.

The biggest immediate impact of the Brexit vote was a spike in the Swiss franc's strength against the euro, prompting Swiss National Bank intervention to rein in a currency whose surge poses a major headache for the export-led economy.

Schneider-Ammann said the government was keeping a close eye on the franc and was in close contact with the SNB.

ECONOMIC ACCORDS AT RISK

Foreign Minister Didier Burkhalter had warned this month the Swiss bid for a deal allowing it to curb immigration could be doomed should Britons vote to leave the bloc.

Schneider-Ammann had lobbied the leaders of Germany, France and Italy for a deal when they attended the opening of the Gotthard rail tunnel this month, but Swiss officials acknowledge the EU will likely be focused on the Brexit fallout.

Swiss diplomats hope it can clinch a deal with Brussels that avoids immigration quotas. The two sides had agreed to hold fast-track talks should Britons vote to remain in the EU.

Implementing unilateral curbs on immigration could torpedo a series of bilateral economic accords that include free movement of people and which stand or fall together.

The Swiss government estimates losing the accords with the country's biggest trading partner could cut output by as much as 7 per cent of gross domestic product by 2035. Business leaders have also said they would find it harder to get qualified staff.

"The agenda of the EU will be certainly very full for the next two years. For Switzerland it will be basically impossible to find a common agreement of how to implement the (quotas)," private bank Bank J Safra Sarasin said.

"A unilateral decision by Switzerland would bear the risk that other bilateral agreements will be cancelled by the EU. This would be very harmful for trade and scientific research in particular and negatively impact potential growth."

Switzerland is the EU's fourth-biggest trading partner, and more than 1 million EU citizens live in Switzerland. - Reuters