North Korea nerves push stocks to worst week since November

World stocks tumbled for a fourth day and were on course for their worst week since November, as the escalating war of words over North Korea drove investors on Friday toward the yen, the Swiss franc and gold. 

Europe's main London, Frankfurt and Paris markets started between 0.5 and 1.1 percent lower and Germany's ultra-safe 10-year government bonds were trading at their highest prices since June. 

U.S. President Donald Trump issued a new verbal warning to Pyongyang on Thursday, saying that his previous promise to unleash "fire and fury" may not have been strong enough after North Korea responded with a threat to land a missile near the U.S. Pacific territory of Guam. 

Japanese markets were closed for a holiday but the in-demand yen powered on, hitting an eight-week high of 108.91 yen to the dollar, adding to its biggest weekly gain since May. 

The yen tends to benefit during times of geopolitical or financial stress as Japan is the world's biggest creditor nation and there is an assumption that Japanese investors there will repatriate funds should a crisis materialize. 

The Swiss franc, the other traditional currency safety-play, has benefited too. Two weeks ago it saw its biggest weakly fall against the euro since the start of 2015. This week has been its biggest rise since June 2016.

Many world stock markets have hit record or multi-year highs in recent weeks, leaving them vulnerable to a sell-off, and the tensions over North Korea have proved the trigger. 

The CBOE Volatility Index, the most widely followed barometer of expected near-term U.S. stock market volatility, hit its highest mark since Nov. 8, when Trump was elected president. 

The Chinese volatility gauge jumped by the most since January 2016 to its highest level in more than seven months. The euro zone's version is the highest since April, when France's election was rattling the region.