U.S. President Donald Trump and China's President Xi Jinping at the G20 leaders summit in Japan on June 29, 2019.

The United States and China hope to sign a phase one trade deal in the first week of January, nearly two years after President Donald Trump launched his trade war, his top trade advisor said Friday.

U.S. Trade Representative Robert Lighthizer, one of the lead negotiators in talks with Chinese officials, also told reporters it would still be wise to be skeptical of whether China would deliver on certain agreements. He also said there would be no new tariffs as long as China negotiates in good faith.

Lighthizer said the signing would be in Washington at the ministerial level and would not involve Trump and Chinese President Xi Jinping.

The Trump administration has not promised a future rollback of tariffs, Lighthizer said, adding it would be wise to be skeptical on whether China would deliver on certain agreements.

Earlier Friday, Trump and Chinese officials announced that the U.S. and China had agreed to the phase one agreement. China agreed to billions of dollars in agricultural purchases from the U.S., while Trump said he would not move ahead with a new round of tariffs on Sunday, among other items.

The USTR said the U.S. will be maintaining 25% tariffs on approximately $250 billion of Chinese imports. The U.S. also agreed to reduce tariffs on $120 billion in products to 7.5% from 15%. That reduction will take effect 30 days after the agreement is signed, Lighthizer said.

Trump had mentioned the tariff levels in a tweet Friday morning in which he also said phase two negotiations would start immediately. Lighthizer also said the negotiations on the next phase in a potential sweeping deal would not wait until after the 2020 U.S. presidential election.

Lighthizer told reporters that China pledged to buy a total of $40 billion in agricultural products, although he said the administration would make its "best efforts" to get to $50 billion, which is the amount Trump himself is calling for.

The phase one agreement was first announced in October. Lighthizer said that the two sides spent the past couple months working out the specifics of the agreement.

The talks, he added, continued until Friday morning — when Trump called to give his approval from the situation room at 10 a.m. ET.

In the opening salvo of his trade war, Trump on Jan. 22, 2018, slapped steep tariffs on imported washing machines and solar energy cells and panels.

— CNBC's Kayla Tausche reported from Washington.