Negotiators from Canada, Mexico and the United States working to update the North American Free Trade Agreement began their discussions on Wednesday -- two days before the fifth round of talks was formally due to begin.

A joint statement did not specify what issues the negotiators are focusing on in the talks, which are due to run through November 21. 

However, it noted that trade ministers from the three countries met on the sidelines of last week's APEC gathering in Vietnam, where they agreed to move forward "as much as possible" on existing proposals.

The ministers agreed to stay away from the new round in Mexico City "so negotiators can continue to make important progress on key chapters advanced in Round 4" held in Washington in October.

US President Donald Trump demanded that the countries renegotiate the 1994 trade pact, which he called a "disaster" for the United States. He has threatened to withdraw from the deal if he is unsatisfied.

Negotiators go into this round amid increased tension over US proposals, including a five-year sunset to NAFTA, which would dissolve the agreement unless the countries renew, and over the US content requirement for autos receiving duty-free status