Sri Lanka and Indonesia must provide more appropriate land on which to build permanent shelter for survivors of last year's tsunami, aid charity Oxfam says.
Mr Clinton pledged to return to Sri Lanka when he visited in May
The appeal came as UN special envoy Bill Clinton arrived in Sri Lanka to review recovery work nearly a year on.
Rebuilding in both countries was too slow, Oxfam said, adding: "New land must be granted to those who lost it."
The tsunami killed more than 200,000 people in 13 countries - 130,000 in Indonesia and 31,000 in Sri Lanka.
Mr Clinton held talks with Sri Lanka's new President, Mahinda Rajapakse on Tuesday.
Mr. Rajapakse praised Mr Clinton for his involvement in tsunami reconstruction.
"Your personal involvement in the post-tsunami reconstruction efforts of our country has been a great source of strength to all of us," Mr Rajapakse said, his spokesman told reporters.
Later Mr Clinton toured parts of the east coast devastated in the 26 December disaster, before heading to Indonesia on Wednesday.
As well as those killed and injured, millions were made homeless along Sri Lanka's southern and eastern coasts.
Thousands in Aceh have not been able to move out of camps
Despite billions of dollars in assistance, the government has been criticised for being too slow to help.
Reconstruction is going on, but along the shoreline many destroyed homes have not been rebuilt because under coastal "buffer zones" they are too close to the sea.
In other cases, the land that victims' homes stood on is now under water or uninhabitable, Oxfam said.
"Thousands of permanent houses have already been built for tsunami survivors but until new land is provided for those made landless, the rebuilding process will be too slow," its director, Barbara Stocking, said.
Oxfam said the Sri Lankan government had made land available but in some cases the land being offered was inappropriate - such as fishing communities being offered land too far away from the sea.
As for Indonesia, the group said the government had not got policies in place to provide new land to the landless.
"This means that in many cases the rebuilding process cannot even start."