MEXICO CITY, March 8 (Xinhua) -- The U.S. plan to impose "seasonal windows" on agricultural produce within the North American Free Trade Agreement (NAFTA) could make future negotiations even harder, said a high-ranking Mexican negotiator on Wednesday.

The United States suggested creating seasonal windows during the fourth round of the NAFTA talks last October, which would restrict agricultural imports from Canada and Mexico and impose tariffs on certain products based on volume and time of year.

Bosco de la Vega, president of Mexico's Agricultural National Council (CNA), told Xinhua that the topic has not been discussed since then due to Mexico and Canada's categorical rejection.

However, de la Vega said he is concerned that the proposal could return to the table and may become another obstacle to the modernization of NAFTA.

The negotiator added the proposal is fully supported by farmers in the U.S. states of Florida, South Carolina and Georgia but is "a non-negotiable topic" for Mexico and Canada. "The danger is that the United States is going to wade into a very complicated area and lengthen it," he said.

The CNA leader said the thorniest topics will be left for last since they could include the involvement of heads of state.

The talks to modernize NAFTA began in August 2017, with the original goal of being finished by the end of the year. However, talks have been prolonged over major differences concerning rules of origin for the automotive industry, among others.

The three countries, Canada, Mexico and the United States, just concluded the seventh round of talks on Monday with only six chapters closed out of 30 under discussion.