China accused the U.S. of “bullying” over its decision to suspend dozens of flights from America by Chinese airlines, underscoring lingering tensions between the world’s biggest economies.

The U.S. “wantonly suspended Chinese flights and disrupted their normal operations,” Foreign Ministry spokesman Zhao Lijian said at a regular press briefing in Beijing on Monday. “It is irresponsible and unreasonable.”

The U.S. on Friday issued restrictions on Air China Ltd., China Eastern Airlines Corp., China Southern Airlines Co. and Xiamen Airlines Co., resulting in the suspension of 44 flights by those carriers. For its part, China has in recent months suspended flights by the major U.S. airlines, citing Covid-19 containment efforts. The U.S. has said that violates a treaty on flight access.

China’s measures to curb virus transmission had been effective, Zhao said. He urged “the U.S. to respect science” and to stop “restricting normal people-to-people exchanges.”

Tension over flights has also escalated due to Chinese requirements on how aircraft are cleaned, as well as quarantine rules for aircrews. The disagreements add to a long list of grievances in the U.S.-China relationship, which deteriorated during the Trump administration and trade war between the two sides.

Disputes have flared in areas including technology imports, a political crackdown in Hong Kong and allegations of human rights violations in Xinjiang, an issue that prompted U.S. President Joe Biden to announce a diplomatic boycott of the Beijing Winter Olympics next month. China denies committing rights abuses in the western region, which has a large Muslim Uyghur population.