Ashton is due to meet Prime Minister Mohamed Ghannouchi as well as officials in charge of political reform and investigations of corruption and abuses of power under the regime of deposed President Zine El Abidine Ben Ali.
Her visit comes as thousands of Tunisians flee the country across the Mediterranean to Italy, mainland Europe's nearest landfall, sparking a row with the Rome government.
Rome has called for its police to be allowed enter Tunisia to prevent a mass exodus of economic migrants, but Tunisia has warned off attempts at "interference" in its internal affairs.
Brussels said Ashton would evaluate the situation on the ground while the EU will also examine possible aid to Italy as it tries to cope with the influx of thousands of Tunisian immigrants on the southern island of Lampedusa.
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