Talks in Montreal later this month to update the North American Free Trade Agreement will be extended by one day, officials said on Thursday, as Mexico and the United States exchanged barbs.

The negotiations have bogged down as Canada and Mexico resist radical demands from the United States to revise the rules governing one of the world's largest trading blocs.

U.S. President Donald Trump is threatening to abandon NAFTA unless major changes are made to the 1994 treaty.

Negotiators had originally been due to meet from Jan. 23 to 28, with U.S. Trade Representative Robert Lighthizer, Mexican Economy Minister Ildefonso Guajardo and Canadian Foreign Minister Chrystia Freeland joining on the final day of the penultimate round of talks. They will now do so on Jan. 29 instead.

Canada's foreign ministry confirmed the shift in dates after Canadian and Mexican sources said the three ministers could not meet on Jan. 28 because of logistical reasons.

Prospects for the talks are unclear. Trump told Reuters on Wednesday that terminating NAFTA would result in the "best deal" to revamp the pact, which he blames for Mexico's large trade surplus with the United States. pnL1N1PD00C]

Trump is also insistent that Mexico pay for a wall he wants to have erected along the southern U.S. border, a demand he repeated on Thursday.

Mexican Finance Minister Finance Minister Jose Antonio Gonzalez Anaya on Thursday reiterated his government’s opposition to the idea.

"Mexico will not pay for a wall and it's not a negotiating stance from Mexico – it's an issue of national sovereignty and dignity," he told reporters after talks with Canadian Finance Minister Bill Morneau in Toronto.

"Our position has been clear from the beginning and will continue to be clear," he added.

Morneau, asked how Canada's government would support businesses if Trump announced the United States was pulling out, said Ottawa was focused on "making sure we continue to be constructive at the table and get to a better NAFTA agreement." Reuters