MEXICO CITY (Reuters) - Mexican President Andres Manuel Lopez Obrador said on Thursday tackling illegal immigration is an issue chiefly for the United States and Central America to address, as a senior Mexican official called U.S. policy on migration “bipolar.”

Speaking after renewed criticism by his U.S. counterpart Donald Trump on Twitter, Lopez Obrador said Mexico would help to check the flow of migrants heading north, but that his country was no longer the main driver of the phenomenon.

Migrants illegally crossing the U.S. border have caused persistent bilateral tension ever since Trump launched his bid for the presidency almost four years ago, saying that Mexico was sending rapists and drug runners into the United States.

With initial campaigning for the 2020 U.S. presidential election already under way, Trump sent out a tweet earlier on Thursday that again attacked Mexico over migration.

“Mexico is doing NOTHING to help stop the flow of illegal immigrants to our Country,” Trump tweeted. “They are all talk and no action. Likewise, Honduras, Guatemala and El Salvador have taken our money for years, and do Nothing.”

Trump also threatened to close the U.S. southern border.

Lopez Obrador was asked during his regular morning news conference about Trump’s tweet, and said he was focused on addressing the root causes of migration.

“We respect president Trump’s position, and we are going to help. That is, this is a problem of the United States, or it’s a problem of the Central American countries. It’s not up to us Mexicans, no,” Lopez Obrador told reporters.

“I just emphasize that migration flows of Mexicans to the United States are very low, a lot lower,” he said. “The Mexican is no longer seeking work in the United States. The majority are inhabitants of our fellow Central American countries.”

Separately, Tonatiuh Guillen, head of Mexico’s National Migration Institute, said at an event in Mexico City the country was caught between U.S. policies that on the one hand were very tough, but on the other, held out the prospect of asylum.

“There’s ... a government structure of bipolar migration policy,” he said. “There are factors of attraction and rejection, and we’re in the sandwich right now.”

Trump’s latest broadside followed calls on social media for another new caravan of migrants to form in Honduras.

His comments came one day after the United States, Honduras, Guatemala and El Salvador agreed to conduct joint police operations in Central America to improve border security and tackle illegal immigration.

The three Central American countries account for the bulk of migrants apprehended trying to cross illegally into the United States.

Over the weekend, a group of around 1,200 migrants, most of them from Central America, began moving toward the U.S. border from southern Mexico.