ASHINGTON/GUATEMALA CITY - US President Donald Trump threatened to deploy the military and close the southern US border on Thursday as more Hondurans and Salvadorans joined thousands of migrants in Guatemala headed north.

"I must, in the strongest of terms, ask Mexico to stop this onslaught - and if unable to do so I will call up the US Military and CLOSE OUR SOUTHERN BORDER!" Trump wrote on Twitter.

Several thousand Honduran migrants traveled this week through Guatemala headed to Mexico, with some hoping to cross into the United States to escape violence and poverty in Central America.

Central American migrants hiked from Honduras through muddy jungle and residential streets, some toting babies along with backpacks, Reuters images show.

In Guatemala City, where migrant shelters filled with people, waves of people departed at daybreak on roads leading to Mexico. The nearest border is about 110 miles (177 km) away.

US Secretary of State Mike Pompeo prepared to travel later in the day to Panama and then Mexico City, where he was to meet with Mexican President Enrique Pena Nieto on Friday.

Mexico's foreign ministry did not immediately respond to a request for comment. Mexico's next foreign minister said he was not surprised by Trump's comments, but did not elaborate.

Trump, who has sought to curtail immigration and build a border wall on the Mexican border, this week threatened to halt aid if Central American governments did not act.

Trump ran for president in 2016 on promises to toughen US immigration policies and build a wall along the US border with Mexico.

Illegal immigration is likely to be a top issue in Nov 6 US congressional elections, when Democrats are seen as having a good chance of gaining control of the House of Representatives from Trump's fellow Republicans.

Frustrated by Congress' failure to fully fund his proposed wall at the border with Mexico, Trump in April ordered National Guard personnel to help the Department of Homeland Security secure the border in four southwestern US states.

In a string of tweets on Thursday, Trump also said the issue was more important to him than the new trade deal with Mexico to replace the North American Free Trade Agreement pact.

"The assault on our country at our Southern Border, including the Criminal elements and DRUGS pouring in, is far more important to me, as President, than Trade or the USMCA. Hopefully Mexico will stop this onslaught at their Northern Border," Trump wrote. He was referring to the United States-Mexico-Canada Agreement, which is awaiting ratification.

The caravan has been growing steadily since it left the violent Honduran city of San Pedro Sula on Saturday. There are no official estimates of the size of the group.

Guatemalan President Jimmy Morales said on Wednesday his government dismissed threatened constraints placed on foreign aid.

He said he had spoken with Honduran President Juan Orlando Hernandez about ensuring the migrants who want to return home can do so safely. 

Reuters