Once the door of subnational cooperation and peoples' friendship is opened, no force can shut it down, Beijing's top envoy in Washington said at an event.

Nine out of 10 U.S. states have had Chinese sister cities or provinces forming a network that has played an important role, like capillaries and cells, in propelling the development of U.S.-China relations, Xie Feng, Chinese Ambassador to the U.S., said at the 5th China-U.S. Sister Cities Conference.

The gathering and its sideline events held at East China's Suzhou city on Friday was attended by 200 representatives from at least 10 Chinese provinces and cities, and nearly 20 county supervisors and city mayors from 22 U.S. states.

Under the theme "Build Green Cities for the People", participants had in-depth exchanges of views, shared experiences, explored opportunities for cooperation and achieved "good results", according to Wang Wenbin, China's Foreign Ministry spokesperson.

The conference reflects the keen interest and strong dynamism in subnational and people-to-people exchanges, and it also shows that cooperation is essential from both countries, Wang said at a briefing in Beijing on Monday.

"China will continue to support and encourage subnational, and people-to-people exchanges. We also hope that the U.S. will work with us to create favorable conditions and the atmosphere for sub-national and people-to-people exchanges and cooperation," Wang said.

The event is part of an increasing number of exchanges between China and the U.S. in recent months, following the visits to China by U.S. officials, including Secretary of State Antony Blinken in June, and a reciprocal trip to Washington by Chinese Foreign Minister Wang Yi last month.

In his speech, Xie noted that since 1979, China and the U.S. have established 284 pairs of sister provinces/states and cities, which have become an important platform for deepening bilateral exchanges and cooperation and promoting mutual understanding and affinity between the two nations.

To see how state-province level exchanges matter, Xie said China is the largest export market for four U.S. states and the top three export markets for 38 states.

Citing his recent trip to Iowa, where Chinese President Xi Jinping visited in 1985, Xie said he personally witnessed how the seeds of friendship sowed by President Xi have grown and flourished, and that subnational exchanges and cooperation enjoy immense potentials and broad prospects.

He said that currently the China-U.S. relationship has seen some positive signs of stabilizing, but still there are "serious difficulties" and "grave challenges" in the bilateral ties.

"It is important to act with a sense of responsibility for history, for the people and for the world, and strive to stabilize and improve China-U.S. relations in accordance with the three principles of mutual respect, peaceful coexistence, and win-win cooperation as outlined by President Xi," the ambassador said.

The envoy called for further tapping the potential of complementarity at the subnational level, strengthening exchanges and cooperation between sister cities, and building more bridges for goodwill between China and the United States.

The efforts will lay a solid foundation and inject impetus into the efforts to bring China-U.S. ties back to the track of sound, stable and sustainable development, he added.