MEXICO CITY (Reuters) - Mexico is preparing to significantly reinforce efforts to detain U.S.-bound migrants who illegally cross its border with Guatemala, in response to a jump in people trying to enter the United States, according to four people familiar with the matter. The U.S. government said this week it is facing the highest number of migrants reaching its border with Mexico in 20 years, presenting authorities a major challenge and sparking concerns inside the Mexican government. The people familiar with the plan said Mexico would deploy security forces to cut the flow of migrants, the bulk of whom come from Central America’s so-called Northern Triangle of Guatemala, El Salvador and Honduras, whose economies were battered by the coronavirus pandemic and hurricanes last year. Two of the people said the National Guard militarized police, which led efforts to bring down the number of illegal immigrants entering Mexico from Central America during an increase in 2019, would be at the fore of the containment drive. “The operations will be more frequent, more continuous and we will be taking part,” from next week, a member of the National Guard said, speaking on condition of anonymity. Mexico’s support is important for U.S. President Joe Biden’s efforts to manage the number of people arriving at the U.S. border, which hit a record for the month in February and is set to tick higher in March, apprehension numbers show. Most people apprehended at the U.S. border are sent back to Mexico. Mexico also needs U.S help on managing the pandemic. It is lobbying hard for Biden to release some COVID-19 vaccine stocks to help with shortages, saying it expects an answer this week. The people did not give details of the new migration plan, but similar operations in the past have focused on catching migrants on a narrow isthmus in the south of the country, rather then trying to stop all crossings on the Guatemalan border where remote and difficult terrain complicates efforts. Migration authorities are already picking up migrants without proper paperwork around the southern city of Tapachula near the border, the National Guard member said. Ivan Porras, an academic studying immigration in Tapachula, said authorities were requesting paperwork from people crossing some usually loosely-controlled sections of the Suchiate river that borders Guatemala. Mexico’s National Migration Institute said it had no information on the matter. Mexico’s National Guard and the office of President Andres Manuel Lopez Obrador did not immediately respond. When asked about the Mexican plan, a U.S. State Department spokesperson said the United States appreciated Mexico’s efforts to stem the flows and aimed to continue close cooperation. The spokesperson declined to comment on specific operations and did not respond to a question on whether the White House had requested such action or knew about it in advance. The White House did not immediately respond to a request for comment. After taking office in 2018, Lopez Obrador created the National Guard to deal with internal security, but ended up deploying thousands of its troopers to curb migration under pressure from the previous U.S. president, Donald Trump, a Republican. Biden, has adopted a more humanitarian approach to migration policy. But the surge in arrivals since he took office in January has tested U.S. authorities and drawn heavy criticism from Republicans. A sense of urgency has mounted within the Biden administration especially over increasing numbers of unaccompanied minors apprehended there. In Washington, a person familiar with the matter said the U.S. administration has seen a growing need for Mexico to do more to secure its southern border but has steered clear of making demands or direct requests for action. Biden’s aides want to avoid Trump’s heavy-handed approach to Mexico, preferring a more respectful tone, but bilateral discussions have touched on how to curb the flow from the Northern Triangle, the source said on condition of anonymity. Mexican officials had anticipated a sharp escalation in immigration under Biden, with one ruefully noting last month that it would force Mexico “to be the bad guys again.”