The United States on Friday took a fresh swipe at a UN migration pact that it shunned a year ago, just days before an international conference in Morocco to endorse the accord.

In a lengthy national statement, the United States said the Global Compact for Safe, Orderly and Regular Migration represents "an effort by the United Nations to advance global governance at the expense of the sovereign right of states to manage their immigration systems."

The non-binding pact agreed in July last year has become a target for right-wing and populist politicians who have denounced it as an affront to national sovereignty.

The United States, which quit negotiations in December 2017, expressed concern that supporters of the migration pact will use it to build on accepted practises and create a "soft law" in the area of migration.

The three-page US statement outlined a number of objections to the document such as a provision stating that detention of migrants should be "a last resort", arguing that this was inconsistent with US law.

The United States is also concerned that the compact "downplays the cost of immigration to destination countries" such as the "loss of employment opportunities" for low-skilled workers and "stresses on public services."

The US statement's release came as the United Nations is preparing to host delegations at a two-day conference in Marrakesh on Monday to endorse the pact, despite a string of defections.

Hungary withdrew from the compact last year and since then Australia, Israel, Poland, Slovakia, the Czech Republic, Austria, Switzerland, Bulgaria, Belgium Latvia, Italy and the Dominican Republic have either quit the pact or expressed strong reservations.

The global pact lays out 23 objectives to open up legal migration and better manage the influx as the number of people on the move worldwide has increased to 250 million, or just over three percent of the world's population.

When the deal was approved in July, it was held up as an example of a UN diplomatic success achieved without the United States at a time when President Donald Trump is questioning the relevance of the world body.

The United Nations has shot back at criticism of the migration pact, insisting that the document is non-legally binding and simply a recognition that international cooperation is needed to address migration.

After Marrakesh, the document will return to the UN General Assembly for approval at a session scheduled for December 19.