WASHINGTON, June 15-- U.S. Secretary of Energy Ernest Moniz on Monday praised the U.S.-China cooperation on clean energy as "one of the most successful international collaborations."

The two countries are in the implementation phase of an expanded clean energy research center, Moniz told a press briefing of the 2015 Energy Conference held by the U.S. Energy Information Administration.

"It will be the first five-year of collaboration. We think this has been really one of the most successful international collaborations," said Moniz.

The clean energy research center (CERC) mentioned by Moniz was set up in 2009 jointly by China and the U.S. government. During its first phase from 2011 to 2015, CERC has become an effective platform for the two countries to strengthen cooperation in advanced coal technologies, building energy efficiency and clean vehicles.

Last November, Chinese President Xi Jinping and his U.S. counterpart, Barack Obama, announced to extend CERC to 2020 and expand the scope of CERC research to include water-related aspects of energy production and use.

Moniz said CERC has established an effective mechanism to use private investments as each government puts into 25 percent of resources and that is matched at least one for one by private sector.

"When we review our work on building energy efficiency, it was very very exciting with tremendous private-sector engagement," Moniz said.

Moniz said the United States and China are going to discuss "specific collaborations" to push forward the new research area announced by the two presidents as both countries have major water challenges that impact energy system.