WASHINGTON, June 24  -- China and the United States will exchange the second offers of negative lists, which outline sectors closed to foreign investment, for negotiations on a bilateral investment treaty (BIT) in early September, a senior Chinese official said on Wednesday.

The two sides discussed their initial offers of negative lists and the next step in BIT negotiations during the seventh round of China-U.S. Strategic and Economic Dialogue (S&ED), China's Vice Finance Minister Zhu Guangyao said at a press briefing after the two countries wrapped up their two-day high-level talks here.

Both sides agreed that it is "a milestone event," but it is necessary to realize that "there's much room for improvement" on the negative lists, Zhu said.

The two countries agreed to exchange their improved negative list offers in early September after intensive negotiations in the coming months, the Chinese official added.

Zhang Xiangchen, China's deputy international trade representative and assistant minister of commerce, said Tuesday that the two sides will have two further rounds of negotiations before September to improve the negative list offers.

Zhu hoped the second negative list offers will have high quality and substantial improvement so that Chinese President Xi Jinping and U.S. President Barack Obama could confirm "major progress" made in BIT talks when Xi pays a state visit to the United States in September.

Then the two presidents could give further clear instructions to both negotiating teams for concluding the talks, Zhu said, noting that "our objective is to conclude the China-U.S. BIT negotiations under Obama administration."

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

But it was not until the 2013 S&ED meetings that the talks entered a substantial phase after the two countries agreed to conduct negotiations on the basis of pre-establishment national treatment with a negative list approach.

The two sides exchanged initial negative list offers at their latest round of talks in Beijing earlier this month, which ushered in a new phase of bilateral negotiations.

The investment treaty is expected to continue to expand two-way trade and investment and cement the foundation of China-U.S. economic ties.