US authorities launched small-scale operations seeking to arrest undocumented immigrants over the weekend in an apparent start to President Donald Trump's vow to launch mass deportation round-ups across the United States.

The operation, which Trump revealed on Twitter last month, then postponed before telling reporters on Friday that it would proceed, was expected to target hundreds of recently arrived families in about 10 cities who had been ordered deported by an immigration judge.

The removal operations are meant to deter a surge in Central American families fleeing poverty and gang violence in their home countries, with many seeking asylum in the United States.

Immigrants and their advocates were bracing for mass arrests, but by Sunday night there were only reports of low-profile operations in a few cities.

"We are doing targeted enforcement actions against specific individuals who have had their day in immigration court and have been ordered removed by an immigration judge," Acting Immigration and Customs Enforcement (ICE) Director Matt Albence told Fox News when asked for an update.

Mary Bauer at the Southern Poverty Law Center (SPLC) said there were no confirmed operations in large Southern cities such as Atlanta.

There also were no reports of arrests from the American Immigration Council, which has lawyers on standby to give legal advice at the country's largest family migrant detention center in Dilley, Texas.New York Mayor Bill de Blasio said there were three ICE operations in his city on Saturday, with no reported arrests. He said there was no ICE activity in New York on Sunday.

In Denver, the Colorado Rapid Response Network of immigration activists said there were unconfirmed reports of ICE or police detaining three people on Sunday in the Potter Highlands area.

Meanwhile, Trump administration yesterday moved to block most migrants who cross the US southern border after passing through Mexico from seeking asylum.

A new rule redefining asylum eligibility -- to take effect today -- is the latest attempt to stem the flow of undocumented migrants into the country, and comes amid White House frustration at Congress's failure to change asylum laws.