US President Donald Trump is expected to sign executive orders that include a temporary ban on most refugees and a suspension of visas for citizens of Syria and six other Middle Eastern and African countries.
Trump, who tweeted that a "big day" is planned on national security on Wednesday, is expected to ban for several months the entry of refugees into the United States, except for religious minorities escaping persecution, until more aggressive vetting is in place.
Another order will block visas being issued to anyone from Syria, Iraq, Iran, Libya, Somalia, Sudan and Yemen, said the aides and experts, who asked not to be identified.
In his tweet late on Tuesday, Trump said: "Big day planned on NATIONAL SECURITY tomorrow. Among many other things, we will build the wall!"
Trump's executive order threatens the refugee resettlement deal with Australia signed late last year, and could leave more than 1,000 asylum seekers in limbo.
Under the deal the US agreed to resettle an unspecified number of refugees being held in Papua New Guinea (PNG) and Nauru on Australia's behalf.
Trump's border security measures probably include directing the construction of a border wall with Mexico and other actions to cut the number of illegal immigrants living in the United States.
The Republican president, who took office last Friday, is expected to sign the first of the orders at the Department of Homeland Security, whose responsibilities include immigration and border security.
On the campaign trail, Trump initially proposed a temporary ban on Muslims entering the United States, which he said would protect Americans from jihadist attacks.
Both Trump and his nominee for attorney general, Senator Jeff Sessions, have since said they would focus the restrictions on countries whose migrants could pose a threat, rather than a ban on those of a specific religion.
To block entry from the designated countries, Trump is likely to tell the State Department to stop issuing visas to people from those nations. He could also instruct US Customs and Border Protection to stop any current visa holders from those countries from entering the United States.
White House spokesman Sean Spicer said on Tuesday the State and Homeland Security Departments would work on the vetting process once Trump's nominee to head the State Department, Rex Tillerson, is installed.
Other measures may include directing all agencies to finish work on a biometric identification system for non-citizens entering and exiting the United States and a crackdown on immigrants fraudulently receiving government benefits, according to the congressional aides and immigration experts.
To restrict illegal immigration, Trump has promised to build a wall on the US-Mexico border and to deport illegal migrants living inside the United States.