Politicians from around the globe will gather Monday in Morocco for a major conference to endorse a United Nations migration pact, despite a string of withdrawals driven by anti-immigrant populism.

The US government disavowed the negotiations late last year, and since then Australia, Austria, the Czech Republic, Dominican Republic, Hungary, Latvia, Poland and Slovakia have pulled out of the process.

The US on Friday took a fresh swipe at the pact, labelling it "an effort by the United Nations to advance global governance at the expense of the sovereign right of states".

But a host of other nations led by German Chancellor Angela Merkel are in Morocco to endorse the deal and the UN remains upbeat that it can help the world better cope with the hot-button issue.

On the eve of the conference, UN special representative for migration Louise Arbour hit back at the pact's critics, insisting the document is not legally binding.

"It is surprising that there has been so much misinformation about what the compact is and what the text actually says," she told reporters in Marrakesh.

"It creates no right to migrate. It places no imposition on states," she said, adding that 159 member countries are due to attend the conference, including "around 100" represented by heads of state, heads of government or ministers.