China has started letting the yuan weaken slightly in a pointed "snub" to U.S. President Donald Trump, Dutch bank ING said in a report.

Different U.S. administrations have long claimed China derives unfair export price advantages by keeping the yuan artificially weak, a charge Beijing denies. Still, the United States has refrained from officially labeling China a "currency manipulator."

Though Chinese authorities have loosened controls over the years, the yuan is still tightly managed and allowed to trade within a narrow band of 2 percent either side of a daily fixed point.

ING said the yuan has been moving much more narrowly in the past two months as the People's Bank of China, the central bank, has emphasized "stability" in currency policy.

But the Chinese currency, also known as the renminbi, weakened against the dollar by 0.2 percent in March, and by 0.3 percent so far in April, ING said on Monday in the report. That compares with the yuan strengthening by 2.45 percent in first two months of the year, it added.