Nineteen US senators, including Democrats and Republicans, sent a letter to Secretary of State John Kerry yesterday expressing "grave concern" that Malaysia may be upgraded in this year's US list of human trafficking offenders.

"A premature upgrade of Malaysia would undermine the integrity of the TIP (Trafficking in Persons) report process and compromise our international efforts to fight human trafficking," they said in the letter, a copy of which was seen by Reuters.

The letter said this year's report is already more than five weeks overdue, which could make it the latest ever released.

Reuters reported last week that the US State Department plans to reverse last year’s downgrade of Malaysia in its annual TIP report, which could smooth the way for a major US-led free-trade deal - the Trans-Pacific Partnership Agreement (TTPA) - with Malaysia and 11 other countries.

Last year, the United States downgraded Malaysia to Tier 3, alongside North Korea and Zimbabwe, citing "limited efforts to improve its flawed victim protection regime" and other problems.

In the letter, the senators called for the integrity of the TIP report ranking process to be upheld, and said an unwarranted upgrade of Malaysia would weaken the credibility of the ranking system.

"Ultimately it would undermine the value of the TIP review process to obtaining meaningful international progress on human rights," the letter said.

Senators signing the letter included Democrats Robert Menendez, Sherrod Brown, Debbie Stabenow, Richard Blumenthal, Edward Markey, Elizabeth Warren, Bernie Sanders, Gary Peters, Jeff Merkley, Robert Casey, Martin Heinrich, Kirsten Gillibrand, Patrick Leahy, Ron Wyden, Amy Klobuchar, Michael Bennet and Al Franken.

Republicans Rob Portman and Marco Rubio also signed the letter.

- Reuters