MEXICO CITY -- A new migrant caravan of about 2,500 people was making its way through southern Mexico on Tuesday, headed for the U.S. border, facing greater heat -- and a much cooler welcome -- than last year's caravans.

The caravan walked past the city of Huixtla in the southern state of Chiapas on Monday, but police were lined up to keep them moving along a highway outside of the town, and did not let them enter -- a contrast to last year, when caravans were allowed to stay in the city centre.

The city said in a statement that it offered water and medical help to the caravan of 2,466 people, mainly from Nicaragua, Cuba, El Salvador, Honduras and Guatemala. It said the caravan included many children in the caravan, and some were suffering in the area's near-100-degree (39 C) heat.

Such caravans aren't getting as a warm a welcome as they did last fall, when local governments and church groups handed out food, water and clothing and police sometimes helped the migrants get rides.

Activists said that the Mexican government was trying to wear the caravans out, or stop them from trying to reach the United States.

"It is a strategy to break them up ... to stop the caravans," said Irineo Mujica of the aid group Pueblo Sin Fronteras, which accompanied last year's caravans and the ones this year.

This year, the Mexican government abruptly stopped issuing "humanitarian" visas at the border with Guatemala. The visas had given migrants legal status while they made their way to the U.S. border.

That decision led a group of migrants, including many Cubans, to rush immigration offices near the border last week. The immigration agency closed those offices in response to the scuffle.

And, following the recent disappearance of 19 migrants in the northern border state of Tamaulipas, police and immigration agents have been increasingly detaining and deporting migrants there. Migrants in Tamaulipas generally use smugglers and are not part of caravans.

In February, a caravan of about 1,600 Central American migrants was confined to an improvised shelter at an old factory building in the northern state of Coahuila.

Migrants were allowed to move freely through Mexico last fall. But Mujica said Mexico had changed its policy "to comply with the expectations of (U.S. President Donald) Trump."