The world's biggest sovereign wealth fund plans to sell down US government debt — bad news for the Treasury market, which needs steady buyers to keep borrowing costs from rising.

• Norway's $2.3 trillion fund wants to cut government bonds from 70% to 50% of its bond portfolio, with US Treasuries falling by roughly a third.
• It is also trimming European government debt and buying more Japanese bonds instead, chasing better returns.
• The sale itself is too small to move rates, but it signals that long-time reliable buyers of US debt are backing away.
• Mohamed El-Erian says the same pressure is showing up in Japan, China, and the Gulf states — the other big holders of American debt.
• Long-term US borrowing rates are already at decade highs as investors worry about the government's growing debt pile.

Outlook: If other big funds and foreign governments follow Norway's lead this fall, borrowing costs for the US will keep climbing.