The number of families crossing the southern U.S. border more than quadrupled in May compared to the same month one year prior - and the number of unaccompanied migrant children more than tripled in the same period, according to federal data.

In May 2018, 14,203 people in families crossed the U.S. border – a 435 percent increase compared to the 2,653 who crossed in May 2017, according to U.S. Customs and Border Protection data.

In addition, a higher proportion of them were entering the country in between official border crossings: About two-thirds of families entering in May 2018 did so illegally, compared to 60 percent in May 2017.

The number of people in families crossing the U.S. southern border rose 435 percent in May 2018 compared to May 2017. This year 14,204 people entered the country via Mexico in May, compared to 2,653 in May 2017

'It's no surprise (families are crossing together) because the word has gotten back that this is the way to get released into the United States,' said Mark Krikorian, executive director of the Center for Immigration Studies, an anti-immigration think tank based in Washington D.C.

A total of 7,235 unaccompanied migrant children entered the U.S. last month – 89 percent illegally. That represents a 329 percent increase from the 1,685 who crossed in May 2017, 87 percent of whom did so illegally.

'If you look at (immigration at the southern border) on an annual basis you will see ebbs and flows depending on the time of year and spring is definitely one of the times of years that we see an increase,' said Lee Williams, vice president at the U.S. Committee for Refugees and Immigrants, a Washington D.C.-based pro-immigration group.

'The reasons haven't changed,' he added. 'Honduras, El Salvador and Guatemala remain some of the most absolutely violent countries in the world. There's a significant amount of gang activity and the governments to varying degrees are unwilling or unable to protect their citizens so they're forced to flee.'

Overall, 339,987 migrants entered the U.S. via the southern border through May of fiscal year 2018 – a nearly 11 percent increase compared to the same period in fiscal year 2017 when 306,793 migrants entered the country. The Customs and Border Protection fiscal year runs October – September.

Nearly three quarters (74 percent) crossed illegally between official border crossings, a slight uptick in the ratio, compared to 73 percent during the same period the year before.

The overall number of people crossing the southern U.S. border has increased over the past 12 months, with 73 percent of border crossers during this period entering the country illegally

'This (illegal crossing) is the result of a bad policy of prosecuting asylum seekers,' said Michele Garnett McKenzie, deputy director at The Advocates for Human Rights, a pro-immigration group. 'We want people to show up to our borders and connect with the authorities, not hide from them.'

It's unclear the extent to which the numbers reflect an increase in people crossing, versus an increase in border patrol enforcement.

'We are in a better position than we were ten years ago with technology and infrastructure,' said Salvador Zamora, a division chief with U.S. Border Patrol. 'If this same volume was coming across ten years ago we probably wouldn't be able to manage it as effectively as we are today,' he said.

The exact number is unknown, but many of those families and children have sought asylum in the U.S. and will be subject to the Trump administration's policies as they enter the immigration court system.

The rate of families and unaccompanied children entering the country increased overall during the past 12 months. A higher proportion of migrants in both categories entered the country illegally

Last week Trump reversed course on his policy of separating immigrant children from parents at the border, and now the administration is seeking legal authority to detain families together.

'We will ... be jailing those families together rather than making sure that they can fairly access the asylum protection system, and they will be held in military bases and large family detention centers,' McKenzie said. 'Locking up families is not an alternative.'

However, Krikorian said he anticipates Trump's reversal will cause a further increase in families entering the U.S.

While the total number of children and people in families known to have entered the U.S. from the southern border has risen year-over-year, their ratio out of overall crossings hasn't changed much during that time.

Children accounted for 11 percent of all entries at the U.S.-Mexico border so far in 2018, compared to about 12 percent during the same period in 2017. The ratio of families rose 1 percentage point year-over-year to account for 28 percent of all southern border crossings through May 2018.