By Jose Torres

MEXICO (Reuters) - A caravan of at least hundreds of migrants left from the southern Mexican city of Tapachula on Sunday, heading for the U.S. southern border.

The smaller caravan plans to join a larger one that left six days ago and is currently stopped about 25 miles (40 km) north in the town of Huixtla.

Organizers said the first had swelled to some 7,000 people while the government in the southern Chiapas state said it estimated the group at 3,500 people.

Many migrants are fleeing poverty and political instability in their homelands, hailing from Cuba, El Salvador, Guatemala, Haiti, and especially Honduras and Venezuela, according to a Reuters witness.

"I think 3.4 months is too long to wait to get a humanitarian visa, to be able to travel through Mexican territory," said Selma Alvarez from Venezuela. "Because we are at the mercy of coyotes, of criminals, it is good that we accompany each other in the caravan, it seems safer to me."

Alvarez added that the group was impatient to get to the U.S. border and start the process to enter the U.S. with appointments secured via a U.S. government app, CBP One, and request asylum.

U.S. President Joe Biden, who is seeking reelection next year, is under pressure to lower the number of people crossing illegally into the U.S. from Mexico.

A record number of people this year have crossed the Darien Gap region connecting Panama and Colombia.

(Reporting by Jose Torres; Writing by Sarah Kinosian; Editing by Josie Kao)