WASHINGTON, May 18 (Reuters) - The Trump administration on Thursday formally launched its effort to renegotiate the North American Free Trade Agreement with Canada and Mexico to try to win better terms for U.S. workers and manufacturers.

With a letter to U.S. lawmakers, U.S. Trade Representative Robert Lighthizer said he triggered a 90-day consultation period with the U.S. Congress and American public that would allow NAFTA talks to begin after Aug 16.

Renegotiation of NAFTA was a key campaign promise of U.S. President Donald Trump, who frequently called the 23-year-old trade pact a "disaster" that has drained U.S. factories and well-paid manufacturing jobs to Mexico.

Lighthizer told reporters that NAFTA has been successful for U.S. agriculture, investment services and the energy sector, but not for manufacturing. He added that he hopes to complete negotiations by the end of 2017.

"As a starting point for negotiations, we should build on what has worked in NAFTA and change and improve what has not," Lighthizer said in a conference call with reporters. "If renegotiations results in a fairer deal for American workers there is value in making the transition to a modernized NAFTA as seamless as possible."

In his letter to congressional leaders, Lighthizer said that NAFTA needs modernization for digital trade, intellectual property rights, labor and environmental standards, rules for state-owned enterprises and food safety standards. (Reporting by David Lawder; Editing by Paul Simao)