China's foreign ministry has complained that US law enforcement officials improperly entered its consulate in Houston, which was ordered to close in an escalating diplomatic feud.
The ministry gave no details but US federal agents checked the consulate's doors and a locksmith was seen working on a lock on Friday after Chinese diplomats left ahead of a 4pm deadline to close.
US-Chinese relations have plunged to their lowest level in decades amid conflicts over trade, technology, Hong Kong, spying accusations and complaints of abuses against Chinese Muslims.
The US government ordered the Houston consulate closed this week, saying Chinese agents tried to steal medical and other research in Texas.
Officials in Beijing responded by ordering the US to close its consulate in the southwest Chinese city of Chengdu.
"As for the US side's forcible entry into the premises of the Chinese Consulate General in Houston, China expresses strong dissatisfaction and resolute opposition," a foreign ministry statement said.
"China will make a proper and necessary response to this."
The statement said that the Houston consulate was Chinese property and that under diplomatic treaties US officials had no right to enter.
In Chengdu, spectators snapped photos outside the US consulate on Saturday as police in T-shirts and surgical masks stood on the footpath and the closed-off street in front of the walled compound.
A bus drove into the compound but spectators saw few other signs of action.
On Friday, a man was detained by police after he set off firecrackers outside the consulate at about 7pm, Chengdu police said on their social media account.
It said the man was given a warning.