Attempts to illegally cross US-Mexico border drops by a third in 2015

The number of undocumented migrants detained near the United States' border with Mexico fell by almost a third in 2015, US government data showed.

Arrests by the Border Patrol fell by 30 percent to 337,117 in fiscal year 2015 -- the second lowest number since 1972, according to data released by the Department of Homeland Security.

It marked an 80 percent drop from the peak reached in 2000.

"Overall this decrease indicates that the investments that we've made in border security are proving effective and that we are seeing fewer attempts to illegally cross the border," said a senior DHS official on condition of anonymity.

Illegal immigration is a highly sensitive issue in American politics, particularly in the Republican camp which is torn between calls for toughness, and attempts to woo Hispanic voters with a more understanding approach.

Republican White House frontrunner Donald Trump has made clamping down on illegal migration a hot-button issue, summed up in an oft-repeated pledge to build a wall along the Mexican border.

Of the 337,117 people apprehended near the US border, 188,122 were Mexican, 57,160 Guatemalan, 43,564 Salvadoran and 33,848 Honduran.

The number of minors intercepted dropped sharply, by 41 percent, to 39,838 in 2015.

A worrying spike in the number of unaccompanied minors crossing the border the previous year had led President Barack Obama to seek Mexico's help in addressing the humanitarian crisis.

Obama's administration moved to reform immigration policy by executive order after attempts at reform through Congress became hopelessly stalled.

The Supreme Court now has to rule on measures enacted by his administration that would shield four million undocumented migrants from deportation.

There are nearly 12 million unauthorized immigrants in the United States, many of them Mexicans who have lived and worked in the country for many years.

According to DHS data, the United States deported a total of 462,463 people from the country in 2015.