China and the United States will focus on the length and quality of "negative lists" - documents that spell out sectors closed to foreign investment - during the next two rounds of negotiations on a bilateral investment treaty, a senior Chinese official said on Tuesday.

"During the latest 19th round of negotiations, the two sides exchanged initial offers of negative lists," Zhang Xiangchen, China's deputy international trade representative and assistant commerce minister, said at a news briefing on the sidelines of the seventh round of the China-US Strategic and Economic Dialogue.

"The negative list itself is a major progress in the negotiations, and both sides described it as a milestone event," Zhang said, adding that it will "fundamentally change the foreign investment administration regime" and "substantially facilitate foreign investment" in China.

"The next issue will be the length and quality of the negative lists" after both sides exchanged initial offers, he said, adding that both countries expressed their willingness to have further progress on bilateral investment treaty negotiations and make the negative lists "shorter and better".

While the negative-list issue is more difficult for China, both sides need to improve their negative-list offers, the Chinese official said, urging the US government to improve its investment environment as more Chinese investments have come into the country.

Zhang said the two sides will have two further rounds of negotiations before September on negative-lists offers, but declined to give a specific timetable for the conclusion of the talks.

"I hope the bilateral investment treaty negotiations could be concluded as quickly as possible," Zhang said.

Adam Posen, president of the Peterson Institute for International Economics, said he believes the US and China can finish the investment treaty talks under the Obama administration.

"There is more room for agreement than people realized," Posen said. "I certainly think (it's) doable before President Obama leaves office in January 2017."

The investment treaty talks began in 2008 as China and the US sought to increase mutual investment, which only accounted for a tiny share of their respective overseas investments.

But it was not until the 2013 Strategic and Economic Dialogue meetings that the talks entered a substantial phase.

Yukon Huang, a senior associate in the Asia Program of the Carnegie Endowment for International Peace, said it is necessary to realize that both sides want to have a bilateral investment treaty, as it will help address a number of investment concerns between the two countries, and investors from both sides will get better access to each other's markets.