President Donald Trump has said asylum seekers must go to ports of entry in order to make a claim.
He says he will issue an executive order next week on immigration.
In a lengthy speech on Thursday, Mr Trump said migrants are abusing the asylum system and making a “mockery” of immigration laws.
Under the Immigration and Nationality Act, migrants claiming asylum are allowed to do so at the border crossings, but also if they cross illegally.
Mr Trump reiterated that he wanted to set up “tent cities” to hold people coming to the US, including those seeking asylum.
The military is working to create space, but nothing has been finalised.
Laws prevent children from being detained beyond 20 days.
The president made his speech amid mid-term election campaigning and with caravans of migrants making their way towards the US border.