The Organisation for Security and Cooperation in Europe (OSCE) launched an observer mission in Turkey on Friday ahead of a vote on expanding President Recep Tayyip Erdogan's powers, but said there were limits on its movement.
The mission follows an invitation from Turkish authorities, the OSCE's Office for Democratic Institutions and Human Rights told reporters in Ankara.
But Tana de Zulueta, head of the mission, admitted there were limitations on freedom of movement.
"We already have limitations in place on freedom of movements for international observers in some parts of the country. We have to take these into account," Zulueta said.
But, she added: "We would like to cover as extensive an area as possible."
Zulueta did not give further details on the "limitations" but the Turkish military has been waging a relentless campaign in the southeast of the country against Kurdish militants.
A team of 11 experts in Ankara and 24 observers from 13 countries will be deployed across the country.
The Turkish public will vote on April 16 on whether to approve constitutional changes that would create an executive presidency.
While the government argues that the changes are necessary for political stability, critics fear it will lead to one-man rule and a further erosion of democracy.
Zulueta said part of the team's job would be assessing the media environment and the ability of Turkish citizens to make an informed choice, amid criticism from commentators that the 'no' campaign was struggling to make its voice heard.
The observers will meet with representatives from civil society, media, political parties and Turkish officials as well as visiting polling stations across the country.
But this will not constitute systematic observation, she added.
The opposition and some NGOs also accuse Ankara of taking advantage of the state of emergency in force since the July 15 failed coup last year to silence any discordant voice through an unprecedented wave of purges.
The goal of the mission will "observe the conduct of these elections in line with OSCE standards and requirements for democratic elections, but also in line with national legislation," Zulueta said.
The day after the referendum, the mission will join with an observer delegation from the parliamentary assembly of the Council of Europe to issue a statement of preliminary findings.