Washington: A coalition of 13 American states are suing President Donald Trump's administration, seeking to block a new rule that would drastically reduce legal immigration by denying visas to poor migrants.

Some experts say the rule could cut legal immigration in half by denying visas and permanent residency to hundreds of thousands of people if they fail to meet high enough income standards or if they receive public assistance such as welfare, food stamps, public housing or Medicaid.

US-bound migrants rest at a camp near an immigration centre in Matamoros, Mexico. Credit:AP

"The Trump administration's message is clear: if you're wealthy you're welcome, if you're poor, you're not," Washington state Attorney-General Bob Ferguson said in a statement. "This rule is un-American, anti-immigrant and unlawful."

The rule, unveiled on Monday and to take effect October 15, expands the definition of a public charge, allowing denials to visa applicants who fail to meet income requirements or who receive public assistance.