MEXICO CITY, Aug 26 (Reuters) - The United States has urged Mexico to clear ad-hoc camps housing thousands of migrants in border cities due to concerns they pose a security risk and attract criminal gangs, officials familiar with the matter said. Facing domestic criticism over a jump in illegal crossings at the U.S.-Mexico border, the administration of U.S. President Joe Biden has pressed Mexico to curb the flow of migrants to help ease pressure on the nearly 2,000 mile (3,200 km) frontier. Two of the biggest camps to have sprung up in northern Mexico are in the city of Reynosa, across the border from McAllen, Texas, and in Tijuana, opposite San Diego, California. Government officials and migrant advocates say the Reynosa camp is home to at least 2,500 people, is unsanitary and has drawn drug gang members looking to recruit desperate migrants. The Tijuana camp is of a similar size, rights groups say. For weeks, the U.S. government has been asking Mexico to clear the camps, in part because the sheer volume of people in them could jeopardize security if they made a sudden rush for the border, two officials familiar with the matter said. The State Department and the White House declined to comment. The U.S. Department of Homeland Security did not respond to requests for comment. Mexico"s Foreign Ministry did not reply to requests for comment. The National Migration Institute declined to comment. The officials emphasized the importance of eradicating conditions that encouraged cartel members to try to extort migrants, or to pressure them into joining their ranks. The U.S. Supreme Court on Tuesday ordered Biden to comply with a Texas-based federal judge"s ruling to revive a Trump administration immigration policy that forced thousands of asylum seekers to stay in Mexico to await U.S. hearings. That has alarmed Mexican officials, who are concerned the country will struggle to cope with more people after the number of apprehensions or expulsions by U.S. agents of migrants crossing the border more than doubled this year. Mexican authorities have stepped up efforts to expel migrants in the country illegally, many from Central America. In the past few weeks it has sent thousands of them to southern Mexico by plane in order to speed up the process.