By Sofia Menchu and Nelson Renteria

GUATEMALA CITY/SAN SALVADOR (Reuters) - Honduras, Guatemala and El Salvador have agreed to join forces and seek support from Mexico to forge a joint strategy in response to Donald Trump winning the U.S. presidency, El Salvador's foreign minister told Reuters on Wednesday.

Trump's election upset has sent shockwaves through Mexico and Central America, which rely heavily on U.S. remittances and bilateral trade.

President-elect Trump romped to victory in the Nov. 8 election by winning over voters with vows to end illegal immigration and re-examine trade treaties that he said have led U.S. firms to ship jobs south to lower-wage economies.

Many of the migrants bound for the United States hail from the poor nations of Honduras, Guatemala and El Salvador, and there are concerns that Trump's promise to deport millions of immigrants would have grave repercussions in Central American countries with few jobs and shaky security.

On Wednesday, the day after a regional meeting in Honduras, the three countries released a joint statement asking their respective foreign ministries to join forces and formulate positions on jobs, investment and migration to deal with the new U.S. administration together, though the statement did not refer to Mexico.

But Honduran President Juan Orlando Hernandez, Guatemala's Jimmy Morales and El Salvador's leader Salvador Sanchez Ceren, have agreed to seek support from Mexico, said Hugo Martinez, El Salvador's foreign minister, confirming what another government source told Reuters earlier.

"What the presidents told us was that aside from this group ... we could expand to look for contact with Mexico, at first, and then also with the other Latin American countries," Martinez said.

Mexico's Foreign Ministry did not immediately reply to a request for comment.

In an interview last week, Humberto Roque Villanueva, Mexico's deputy interior minister for migration, said he expected deportations of undocumented Mexican migrants in the United States to start rising when Trump takes office Jan. 20 but the process will not begin soon.

Trump has repeatedly said he plans to build a wall along the U.S.-Mexican border, insisting that Mexico will pay for it.

PLAN IN ACTION

The first meeting between the Central American foreign ministers will take place on Thursday in San Salvador, sources said, adding that there is no date yet for when Mexico might join.

The Central American foreign ministries said they were already looking to strengthen their consular services in the United States and preparing a network of activists, lawyers, non-profits and church leaders to help safeguard against any possible deportation wave.

Maria Andrea Matamoros, Honduras' deputy foreign minister, told Reuters she expected the number of migrants bound for the United States to rise before Trump takes office in January.

"(The wall) is the perfect advertising campaign for a human trafficker, and now, with the election of Trump, that has magnified, and we're already seeing - incredibly - a rise in the flow of migrants," she said.

On Tuesday, Honduras, Guatemala and El Salvador launched a joint security system to combat gangs and drug smuggling, as well as curtail migration and lower murders in one of the world's most violent regions.

The joint security force is part of the so-called "Alliance for Prosperity" plan, which aims to regenerate the region with support from Washington, creating jobs and infrastructure.

That plan was hatched after a sharp rise in Central American child migrants bound for the United States sparked a crisis for outgoing U.S. President Barack Obama in 2014.

Speaking at the event in the western Honduran city of Nueva Ocotepeque on Tuesday, President Hernandez said he would meet U.S. lawmakers from both parties to defend the Alliance for Prosperity.

(Additional reporting by Gustavo Palencia in Honduras and Gabriel Stargardter in Mexico, Writing by Enrique Pretel; Editing by Gabriel Stargardter, Christian Plumb and Lisa Shumaker)