SYDNEY, Australia — Australia and America will swap asylum seekers under a contentious scheme to deter migrants from seeking asylum in either country.

Under the exchange scheme, asylum seekers will lose the chance to choose their destination. The boat people held by Australia on the remote Pacific island of Nauru will be sent to America, while Cuban and Haitian refugees held at Guantanamo Bay in Cuba will be sent to Australia.

The plan expands Australia’s policy of dispatching Asian and Middle Eastern boat people to other countries, including Nauru, Papua New Guinea, and Indonesia. Opposition politicians and human-rights groups said that far from deterring refugees, the plan could end up rewarding them with a new life in America.

The two countries signed a legally binding memorandum of agreement on Tuesday, and the first exchanges could start within months.

It is likely that the first batch to be swapped will be 83 Sri Lankans and eight Burmese, who were intercepted by the Royal Australian Navy and sent to Nauru. Under the agreement, the two countries would swap 400 asylum seekers — 200 from each country — this year and in 2008, the policy will be reviewed in 2009.

Prime Minister Howard insisted the scheme would deter boat people from trying to reach Australia. “It is part of our policy to reinforce the message to those who would engage in people smuggling that this country has a very tough border protection policy,” he said.

However, opposition politicians and refugee rights groups said the plan defied logic because it would make Australia a stepping stone to America.

Tony Burke of the opposition Labour Party said the plan would encourage a new wave of refugees to pay smugglers to take them to Christmas Island, a remote Australian territory in the Indian Ocean and a favorite drop-off point. Kerry Nettle, a senator in the Green Party, described the plan as “bizarre.”