MEXICO CITY (AFP) - US President Barack Obama arrived in Mexico on Thursday to put trade back at the heart of bilateral ties, but his southern neighbor's shifting drug war tactics loom large over the visit.

Mexican President Enrique Pena Nieto hosts Mr Obama on the first stop of a three-day trip that will also take him to Costa Rica for a summit with Central American leaders, with trade, United States (US) immigration reform and the battle against drug cartels high on the agenda.

Security was tight for Mr Obama's visit, with the international airport's runways closed for half an hour for Air Force One's landing.

The president headed first to the historic National Palace, famous for its Diego Rivera murals, for talks with Pena Nieto.

After almost seven years of bloodshed by drug gangs that has left 70,000 people dead in Mexico, Mr Pena Nieto and Mr Obama have both made clear they want to turn the spotlight back on trade ties and other matters.

Twenty years into the North American Free Trade Agreement (Nafta), also including Canada, Mexico is Washington's third-ranked trade partner, with US$500 billion (S$616 billion) exchanged every year.

Mexico and Washington want to "talk about the benefits and the need to re-balance and diversify the relationship," Mr Sergio Alcocer, Mexico's deputy foreign minister for North America, told AFP.

Before Mr Obama's departure, the US announced the creation of a bilateral forum on higher education, innovation and research to broaden educational exchanges.

But the relationship has been marked by deep cooperation in the fight against powerful drug cartels that make billions of dollars by feeding cocaine, marijuana and heroin to US addicts.

The US is providing US$1.9 billion in aid, including police training and crime-fighting equipment, to help Mexico fight the drug gangs.

Mr Pena Nieto, who visited Mr Obama in Washington shortly before taking office in December, wants to refocus the drug war on reducing the wave of violence plaguing his country.

While he is keeping troops that were deployed in the streets by his predecessor for now, his new strategy includes a crime prevention program and a shift in the way Mexico will work with US law enforcement.

His predecessor, Mr Felipe Calderon, forged unprecedented security ties with Washington by allowing US agencies to deal directly with Mexican counterparts during his six-year administration.

But the new government wants to channel all security matters through a "one-stop window:" the powerful interior ministry, which has been tasked with coordinating Mexico's fight against organized crime.

"It is not right, and it will not be possible or allowed for each agency to determine whom they want to deal with," said Interior Minister Miguel Angel Osorio Chong.

"Now we must know what each one of us wants, and we have to control it. You can't do things unilaterally, looking at the interests of one country," he said. "We are going to coordinate and agree on things."

Mr Obama said on Tuesday that he was not yet ready to judge how Mr Pena Nieto's strategy would change security relations until he had spoken to him.

"At this point, we're confident that we're going to have a good constructive and effective security relationship with Mexico, and we look forward to hearing from them about how they plan to go forward with it," said Mr Obama's advisor for Latin America, Mr Ricardo Zuniga.

Rights groups want Mr Obama to address the high level of impunity in Mexico, with Reporters Without Borders urging the US leader to commit to helping "restore the rule of law and civil liberties" in a country where 86 journalists have been killed in the past 10 years.

After holding talks at the National Palace, Mr Pena Nieto will host a dinner for Mr Obama at the Los Pinos presidential residence.

Mr Obama will deliver a speech at the Anthropology Museum on Friday before heading to San Jose for a summit with Central American leaders. He returns to Washington on Saturday.

With 11 million undocumented migrants living in the US - two-thirds of them from Mexico - regional leaders will want to discuss Mr Obama's push for comprehensive immigration reform.

Some 500 people held a protest outside the US embassy in Mexico City before Mr Obama's arrival, demanding that the US immigration reform include a provision allowing relatives of migrants to join their families across the border.