Inversion is spreading across the US yield curve, where short-dated yields are running above long-dated ones, which has also unsettled investors as yield curve inversion often precedes a recession.
"A deeper inversion is sending a stronger statement that a meaningful slowdown is coming," said Brian Rehling, co-head of global fixed income strategy at Wells Fargo Investment Institute in St. Louis, Missouri. "A recession is a possibility in the next 12 to 18 months, but it's not a done deal."
Investors added to their safe-haven holdings of Treasuries as UK Prime Minister Boris Johnson sought to limit parliament's opportunity to derail his Brexit plan by suspending the House of Commons for around a month, starting in mid-September.
Treasury prices pared their gains as Wall Street rose, reversing earlier losses.
While some fund managers view Treasuries as expensive, they are hard pressed to make a case to sell them given the uncertain outcome of the trade developments between Beijing and Washington.
"It's hard to see where the endgame is with the trade tensions," said James Barnes, director of fixed income at The Bryn Mawr Trust Co. in Devon, Pennsylvania.
The Federal Reserve is also monitoring the trade tensions in its economic outlook.
Interest rates futures implied traders fully expect the US central bank to lower key borrowing costs by at least a quarter point at its Sept. 17-18 policy meeting, following up on its first rate cut since 2008.
Meanwhile, the Treasury Department sold $18 billion in two-year floating-rate notes and five-year fixed-rate debt to solid demand.
It will complete this week's $113 billion of fixed-rate government note supply with a $32 billion sale of seven-year debt on Thursday.
In late Wednesday trading, the yields on 30-year government bonds were 1.939pc, down 2.2 basis points from late Tuesday. They hit an all-time low of 1.905pc earlier Wednesday.
The 30-year yield is below 3-month T-bill rates, which has not happened since 2007.
As for the rest of the yield curve, the spread on three-month T-bill rates over 10-year yields widened to as much as 55 basis points, a level not seen since March 2007, while the premium on 2-year yields above 10-year yields increased to as high as 6.5 basis points, according to Refinitiv and Tradeweb data.