Tuxtla Gutiérrez (Mexico) (AFP) - Authorities blocked a new caravan of Central American migrants Wednesday after they entered Mexico bound for the United States, as the government scrambled to dodge President Donald Trump's threat to impose tariffs over undocumented immigration.

Soldiers and police forced hundreds of migrants in the group -- which was mostly from Honduras -- to a halt in the southern town of Metapa de Dominguez, about 12 kilometers (7.5 miles) from the Mexican-Guatemalan border.

The National Migration Institute (INM) said about 420 migrants had been stopped and taken to a detention center by bus.

But many others may have fled: state police initially estimated the caravan had some 1,200 people.

The incident came as a high-level delegation led by Mexican Foreign Minister Marcelo Ebrard held talks in Washington with US officials in a bid to avoid Trump's threatened tariffs.

Trump is threatening to apply tariffs of five percent on all Mexican exports starting Monday, and rising incrementally to 25 percent by October.

That has triggered panic in Mexico, which sends 80 percent of its exports to the United States.

The talks, which broke off Wednesday without agreement, are due to resume Thursday. Trump tweeted that if no deal is reached then, the tariffs will come into force Monday.

The INM said that where applicable, the detained migrants would be "assisted to return to their countries of origin" -- its term for deportation.

Migrant rights activists traveling with the group told AFP more than 100 members of Mexico's newly created National Guard took part in the operation.

Tension flared as officers brandishing anti-riot shields blocked the migrants' path, the activists said. But despite a shoving match, the authorities managed to force the caravan to a stop.

Separately, migrant rights group People Without Borders (Pueblo Sin Fronteras) said two of its members had also been detained: its leader, Irineo Mujica, in the northern state of Sonora, and another activist, Cristobal Sanchez, in Mexico City.

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Mexico has deployed the National Guard to the southern border and stepped up detentions and deportations in a bid to slow the flow of migrants crossing its territory toward the United States.

But migrant detentions at the US-Mexican border still increased by 32 percent month-on-month in May, to more than 144,000, according to US Customs and Border Protection.

Tension has soared between the US and Mexico over the surge of migrants in recent months -- mainly Central Americans fleeing poverty and violence.