MEXICO CITY (Reuters) - Mexico’s president is expected to congratulate U.S. President-elect Joe Biden next week on his victory once it is certified, three officials told Reuters, after weeks of waiting that have upset allies of the incoming U.S. leader. President Andres Manuel Lopez Obrador is among a handful of world leaders yet to recognize Biden’s election victory over Republican President Donald Trump, including Russia’s Vladimir Putin and Brazil’s Jair Bolsonaro. Lopez Obrador aims to send Biden his congratulations the day after the U.S. Electoral College on Dec. 14 votes to certify the election, the government sources said. The Electoral College vote has historically been a formality. This year it took on outsized importance after Trump and his lawyers sought to overturn Biden’s win with unsubstantiated claims of election fraud. Judges dismissed those claims time and again due to lack of evidence. Biden, a Democratic former vice president and long-time senator, will formally assume the presidency on Jan. 20. Roberto Velasco, a senior Mexican diplomat responsible for North America, went to Washington last week and held meetings aimed at making inroads with Biden’s Democrats, according to two other sources with knowledge of the visit. Velasco did not hold formal meetings with Biden transition officials, one of the sources said. A Democratic congressional aide confirmed that he had met with Velasco. Among major bilateral issues for Mexico as it seeks to engage the new Democratic administration will be immigration, trade and the environment, the source said. Lopez Obrador’s office and the foreign ministry did not immediately respond to requests for comment about the plans or Velasco’s visit. In a newspaper column referring to the visit published on Tuesday, Velasco wrote that he met with Senate and House of Representatives advisors on the foreign relations committees. He did not mention the transition or Biden. Since taking office two years ago, Lopez Obrador has sought to build a constructive relationship with Trump, who angered many in Mexico during his 2015-16 presidential campaign by labeling Mexican migrants rapists and drug runners. As president, the Mexican leftist has gone out of his way to avoid confrontation with Trump, in marked contrast to his time in opposition, when he likened the American’s migration policies to Adolf Hitler’s treatment of Jews in Nazi Germany. Trump last year threatened to slap tariffs on all Mexican exports if Lopez Obrador did not stem the flow of mostly Central American migrants headed to the U.S. border. In response, Mexico deployed tens of thousands of national guard troopers to its northern and southern borders. Since then political tensions and migrant crossings have lessened. Mexican officials say Lopez Obrador has held back from recognizing Biden’s victory amid Trump’s claims of fraud to avoid provoking the departing U.S. president. Friction has risen between the two countries in recent weeks following the October arrest and subsequent release of Mexico’s former defense minister Salvador Cienfuegos. In a retaliatory move that could be an early test for diplomacy under Biden, Lopez Obrador has sent a bill to Congress aimed at tightening up the rules of how foreign agents, such as those from the U.S. Drug Enforcement Administration, operate in Mexico.