Polls showed France's presidential election campaign tightening further as financial markets fretted about the rising popularity of far-left candidate Jean-Luc Melenchon. 

Investors have long been anxious about Marine Le Pen, leader of the far-right National Front, who has promised a referendum on whether to quit the European Union and ditch the euro if she assumes office.

She has been joined on the list of investors' concerns by far-left veteran Jean-Luc Melenchon, who has surged in the polls after strong performances in two candidates' debates.

The Communist-backed Melenchon also wants a referendum on EU membership after an attempt to renegotiate the EU treaties.

The latest Ifop-Fiducial poll showed Ms Le Pen winning 23.5% in the first round on 23 April, one point ahead of centrist Emmanuel Macron.

Both Ms Le Pen and Mr Macron's support dipped by half a point from yesterday, while conservative Francois Fillon was stable on 19% and Mr Melenchon unchanged on 18.5%.

The top two candidates go through to a run-off on 7 May,where polls say Mr Macron would easily prevail.

Traders cited the French election, as well as US relations with Syria and North Korea, as reasons why investors switched to safe assets, such as gold or US Treasuries.

Concern about a Le Pen victory, which would put further pressure on the EU after Britain's decision to leave the bloc, led to an unusual foray into French politics by Germany, France's traditional partner at the heart of the EU.

"We need a pro-European France," German Finance Minister Wolfgang Schaeuble said in Berlin.

"I hope Le Pen does not become French president," he added.

Outgoing Socialist President Francois Hollande is also concerned about Mr Melenchon's rising popularity, according to Le Monde newspaper, and this has fed speculation he could endorse Mr Macron as the best hope to win rather than official Socialist candidate Benoit Hamon, who is doing poorly in the polls.