Violence in southern Yemen has marred an election to replace veteran leader Ali Abdullah Saleh.
At least nine people, including a child, were killed in violence which had been widely expected after separatists called for a voter boycott.
Half of Aden's polling stations closed early as a result but voting in the capital, Sanaa, was calm and orderly.
A US spokeswoman said the country was "encouraged" by the "very strong and positive referendum".
"We congratulate the Yemenis for really launching this process, taking ownership of it as a population, and we will stand with them as they take the next steps," state department spokeswoman Victoria Nuland said.
The poll comes a year after violent anti-government protests erupted in the Arabian peninsula's poorest country.
Voters are due to rubber-stamp Vice-President Abdrabbuh Mansour Hadi - the only candidate.