Aerial view of Salvadorean migrants heading in a caravan to the US, crossing the Suchiate River to Mexico, from Ciudad Tecun Uman, Guatemala. AFP

Mexico's immigration centers are becoming increasingly squalid and overcrowded as authorities step up the detention of migrants headed for the United States, with inmates languishing for weeks amid medical neglect, according to detainees, lawyers and rights groups.

At least 380 Latin American migrants have died on their journeys this year, many of them Venezuelans drowning in the Caribbean or Central Americans perishing while trying to cross the US-Mexico border, the U.N. migration agency said.

The toll, 50 percent more than the 241 recorded as of mid-June 2018, also coincides with tightened security along the US southern border, which often leads migrants to turn to underground criminal smugglers and take riskier routes, it said.

President Donald Trump has made reducing illegal migration one of his signature policy pledges. His administration on Monday cut hundreds of millions of dollars in aid to El Salvador, Guatemala and Honduras, after Trump blasted the three Central American countries because thousands of their citizens had sought asylum at the U.S. border with Mexico.

"This month has been marked by several tragedies on the US-Mexico border, where at least 23 people have died since May 30 May, that is more than one per day," spokesman Joel Millman of the International Organization for Migration told a briefing.

IOM figures show that so far, 144 migrants are known to have died in Mexico, 143 in the Caribbean, 66 along Mexico's southern border with Central America and 27 in South America.

A further 42 reported deathss were under investigation in Mexico, and of several dozen more refugees and migrants crossing the Darien jungle in Panama, he added.

"So we are seeing a level of fatality that we haven't seen before. We caution that with the summer months just beginning, with the intense heat that brings, we can expect it to get worse," Millman said.

Four million Venezuelans have fled their homeland, most of them since an economic and humanitarian crisis began in 2015, the UN refugee agency says. Most went overland to Colombia, Peru, Ecuador and Brazil.

But in images reminiscent of desperate Cubans fleeing their homeland in decades past, Venezuelans increasingly are taking to the sea in rickety boats.

The overall toll includes more than 80 Venezuelans who have died or disappeared in three shipwrecks in the Caribbean in the past two months, Millman said.

Reporters spoke to more than a dozen recent detainees at the Siglo XXI detention center, the country's largest. They described being held in the facility in Chiapas state on Mexico's southern border for long periods without information about their cases.

The detainees reported severe overcrowding, sparse water and food, and limited healthcare.

Their accounts were supported by two lawyers representing 26 other inmates, as well as the migration ombudsman at Mexico's National Human Rights Commission and reports from two migrant rights groups: Fray Matias de Cordova and the Human Rights Observation Mission for the Refugee and Humanitarian Crisis in Southeast Mexico, a collective of 24 aid groups.

Reuters