Treasury bonds exchange traded funds have surged this year, with Treasuries enjoying their best start to a year since 2003, as global economic weakness and increased uncertainty, such as the upcoming “Brexit” vote, bolstered demand for safe-havens.

Among the best performing Treasury bond ETFs so far this year, he iShares 20+ Year Treasury Bond ETF (NYSEArca: TLT) gained 14.3%,  PIMCO 25+ Year Zero Coupon US Treasury (NYSEArca: ZROZ) increased 21.7% and Vanguard Extended Duration Treasury ETF (NYSEArca: EDV) advanced 20.7%.

Related: ETFs to Watch as Brexit Uncertainty Mounts

The yield on benchmark 10-yer Treasury notes inched higher Friday to 1.618% after dipping to a low of 1.518% Thursday, its lowest level since 2012.

According to the Bank of America Corp.’s U.S. Treasury Index, Treasuries have returned 4.8%, the most at this period of the year since 2003, as traders anticipated a falling likelihood of a Federal Reserve interest rate hike, with the probability of a move this year dipping to about 49% from 74% at the end of May, Bloomberg reports.

U.S. government bonds rallied this month after a June 3 report revealed the weakest job creation in almost six years, diminishing the chances the Fed would hike rates on a rebounding economy.

Since the weak economic report, global economies have been anxiously waiting on the prospects of a so-called Brexit or the United Kingdom’s referendum on whether or not to stay within the European Union. The Federal Reserve and Bank of Japan even cited the referendum as a factor in delaying further action in monetary policies.

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