US President Joe Biden is coming under increasing pressure to respond to a growing crisis at the border, as the numbers of migrants fleeing Central America shows no signs of abating. Mr Biden on Monday sent two top White House officials to Mexico and Guatemala for talks in his latest attempt to stop illegal migration. Democrats and Republicans are raising the alarm on the issue. Kyrsten Sinema, a Democratic senator from Arizona, and John Cornyn, a Republican senator from Texas, on Monday wrote to Mr Biden urging him to use his “full authorities” to respond to the “crisis” at the border. They said immediate action was needed to ensure there is enough space to house migrants and to improve the asylum process. The surge has put Mr Biden in a bind. During the 2020 campaign, he promised to satisfy liberal activists by establishing a more humane immigration system. But that pledge has come under fire from conservatives, and some Democrats from border states, who have demanded that he send a firmer message to migrants seeking asylum that they are not welcome. The number of border crossings is especially pronounced among unaccompanied children and teenagers, creating a humanitarian predicament and political problems for the White House. Mr Biden said on Sunday that he planned to visit the US-Mexico border “at some point” for a first-hand look at conditions. White House Press Secretary Jen Psaki on Monday said the US had increased warnings to people in Central America not to come. Ms Psaki said there had been more than 17,000 radio advertisements aired in the region by the State Department. At the same time, she denied that the situation amounted to a crisis. “Children presenting at our border who are fleeing violence, who are fleeing persecution, who are fleeing terrible situations, is not a crisis,” Ms Psaki said. “We feel that it is our responsibility to humanely approach this circumstance and make sure they are treated and put into conditions that are safe.” But unaccompanied minors are arriving in greater numbers and have overwhelmed some holding centres at the south-west border. Photos released on Monday by Henry Cuellar, a Democratic representative from Texas, showed migrants huddled together in crowded conditions inside an overflow centre in Donna, Texas, as space runs short in shelters for children and families. The envoys’ trip shows growing concern in the administration. Roberta Jacobson, the co-ordinator for south-west border affairs, and National Security Council director, Juan Gonzalez, will meet senior leaders to “develop an effective and humane plan of action to manage migration”, council spokeswoman Emily Horne said. The US delegation will meet Mexican Foreign Minister Marcelo Ebrard, deputy secretary for North America Roberto Velasco and other foreign ministry officials, a senior administration official said. In Guatemala, President Alejandro Giammattei and Foreign Minister Pedro Brolo will host Mr Gonzalez and Ricardo Zuniga, a State Department official who is being named special envoy to the northern triangle region of Central America. Ms Jacobson and Mr Gonzalez plan to discuss ways to stop the migrants from travelling to the US border and ways to address the root causes of migration, such as corruption, violence and poor economic conditions in Honduras, El Salvador and Guatemala. The US officials in Mexico will pitch a joint development strategy for southern Mexico and the northern triangle to boost job creation and give potential migrants better reasons to stay home, the official said. Guatemalan civil society leaders and representatives of non-government organisations are also due to meet the US delegation. The Biden administration included $4 billion in aid for Guatemala, Honduras and El Salvador in its proposal for an immigration overhaul, but Ms Jacobson has said the money will be contingent on those governments rooting out corruption. The US has seen a surge in the number of people encountered by border officials in recent weeks. That includes 18,945 family members encountered in February alone, an increase of 168 per cent from January, the Pew Research Centre said. Republicans have accused Mr Biden of creating a crisis by repealing some of former president Donald Trump’s hard-line policies while using more welcoming speech on immigration. “The border right now is wide open because the Biden administration dismantled the very effective policies of the Trump administration and the agreements we had with Mexico and other Latin American countries,” Tom Cotton, a senator from Arkansas, told Fox News Sunday. Mr Biden and his top aides said they were still turning back by far most of those who show up at the southern border, using Title 42, a provision enacted to stem the spread of coronavirus. But his decision to accept unaccompanied minors has strained shelter space, which was already in shorter supply due to coronavirus restrictions. The Department of Health and Human Services, which operates the shelters, had more than 9,500 children and teens in custody as of last week. Another 4,500 minors were in centres run by Customs and Border Protection. Minors are only supposed to stay at the sites for 72 hours but many have stayed longer due to lack of space at shelters. Immigration and Customs Enforcement said at the weekend it that would add a temporary shelter with 1,239 beds for families who arrived at the border. A small number of families with small children are not expelled immediately under the pandemic rule and are placed into immigration proceedings in the US, putting further strain on space in facilities. “Let’s get these kids out of custody,” Vice President Kamala Harris said in Jacksonville, Florida, on Monday. “We’ve got to treat this issue in a way that is reflective of our values as Americans and do it in a way that is fair and is humane. "But we have to meet the moment, and there’s a lot of work going into that.”