China threatened unspecified “strong countermeasures” if the U.S. Congress enacts legislation supporting Hong Kong protesters, in a sign of the deepening strain between the world’s two largest economies as they attempt to seal a trade deal.

China’s foreign ministry issued the warning Wednesday after the U.S. House passed a package of measures backing a pro-democracy movement that has rocked the former British colony for more than four months. Among them was the Hong Kong Human Rights and Democracy Act, which subjects the city’s special U.S. trading status to annual reviews and provides for sanctions against officials deemed responsible for undermining its “fundamental freedoms and autonomy.”

The threat roiled markets during Asian trading, at one point wiping out a 0.8% rally in the regional equity benchmark. Futures on the S&P 500 Index were still down 0.2% as of 8:06 a.m. in London after falling twice that much in Asia. The yuan was also 0.2% lower against the dollar offshore.

While the legislation must also pass the U.S. Senate and be signed by President Donald Trump to become law, it already has strong bipartisan support in the Republican-run upper chamber. The Hong Kong measures were passed by the Democrat-controlled House by unanimous voice votes Tuesday.