MEXICO CITY (Reuters) – Mexico’s government expressed fresh concern to the United States on Friday about delays caused by the Trump administration in the movement of goods and people across the U.S.-Mexico border, saying it is hurting trade.

Mexico’s foreign ministry said it presented a diplomatic note to the U.S. embassy in Mexico on the matter, the latest sign of tension in relations with the United States.

The delays have occurred since U.S. President Donald Trump last month threatened to close the border if Mexico does not stop illegal immigrants from reaching the United States.

“The foreign ministry signalled the urgency of improving the movement of goods and people, and of deepening mutual cooperation to guarantee the efficiency and safety of our shared border,” the ministry said in a statement.

After Trump’s threat, U.S. border agents were moved to handle the influx of migrants, causing delays.

The Mexican government has already expressed concerns over delays, saying this week it is vital to speed up the flow of goods and people on the border.

In spite of Trump’s warnings and the likelihood that migrants at the U.S.-Mexico border face months of waiting to have U.S. asylum requests processed, the flow of people persists.

Hundreds of migrants boarded a northbound freight train in southern Mexico late on Thursday, frustrated by efforts by the Mexican government to slow their progress.

Meanwhile, about 1,300 migrants escaped a holding centre near the southern border with Guatemala on Thursday night. Most later returned to the centre, Mexico’s government said.