President Xi Jinping sent a congratulatory letter to World Environment Day celebrations on Wednesday, calling for joint efforts to promote green, low-carbon and sustainable development.

"We have only one Earth. Protecting the ecological environment and pushing sustainable development are our joint obligations," Xi said in the letter.

While the international community is actively pushing for implementation of the UN 2030 Agenda for Sustainable Development, countries are facing severe challenges, including environmental pollution, climate change and declining biodiversity, Xi said.

He said China has attached great importance to environmental protection, based on the understanding that lucid waters and lush mountains are invaluable assets, and sought harmony between humans and nature.

World Environment Day, celebrated in more than 100 countries, is the United Nations' day for encouraging international awareness and action to protect the environment.

Every World Environment Day has a different host country, and this year's host is China, with the official celebration taking place in Hangzhou on Wednesday. Present were senior officials including Vice-Premier Han Zheng and Joyce Msuya, acting executive director of the UN Environment Programme.

This year's theme is air pollution, and the UN is calling on governments, industry, communities and individuals to explore renewable energy and green technologies and improve the air quality in cities and regions across the world.

In a story on the UN News website, China was praised by the organization for the "significant strides" it has made to improve air quality.

The fight against haze in Beijing was particularly praised by the organization. "The concentration of fine particulates in the air has fallen by a third over the past two decades, beating the target set by the Chinese State Council," it wrote.

"Beijing has achieved impressive air-quality improvements in a short amount of time," said Dechen Tsering, director of the UN Environment Regional Office for Asia and the Pacific. "It is a good example of how a large city in a developing country can balance environmental protection and economic growth," she said.

In his congratulatory letter, Xi also mentioned that China has made remarkable progress in building a "beautiful China", and would like to work with other countries on the path of green growth and jointly build a "beautiful world".

Xinhua contributed to this story.