Mexico has 6,000 members of its National Guard stationed on the border with Guatemala to stem the flow of migrants on their way to the US.
Mexican foreign minister Marcelo Ebrard is trying in Washington to prevent the US from levying tariffs on Mexican goods for illegal migration.
US President Donald Trump vowed to raise import duties of 5% on Monday if no agreement is reached.
On Wednesday, "not nearly enough" progress has been made.
After the recent talks on Thursday, Mr. Ebrard said he was "an optimistic man".
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He said armed forces would be stationed at Guatemala's southern border to prevent migrants from entering the country.
"We stated that there are 6,000 men and that they are used there," he said, adding that the talks would continue on Friday.
What is Trump threatening?
According to the US President's proposal, tariffs on goods such as cars, beer, tequila, fruits and vegetables would increase by 5% each month and reach 25% by October. Mostly Central American migrants entering the US this year have applied for.
The president announced the planned Mexico tariffs on Twitter last week.
What is the situation at the border between the US and Mexico?
US Customs and Border Guard announced this week that migrants' arrests in May had risen to their highest levels in more than a decade.
Border Patrol arrested 132,887 migrants who attempted to enter the US from Mexico in May, an increase of 33% from the previous month.
There were 84,542 families and 11,507 unaccompanied children.
The arrests were the highest monthly total since Mr. Trump took office.
Another 11,391 migrants were classified as "inadmissible" and rejected after arriving at US ports of entry.
The system is broken, "said reigning CBP Commissioner John Sanders.
What are the numbers compared to previous years?
Official figures show that the number of illegal border crossings since 2000 has dropped below 2018 400,000.
In 2017, Mr. Trump's first year in office, the numbers were the lowest since 1971.
However, in recent years the number of arrests has risen again, especially in recent months.