"I have not made a decision on the second term yet", he told a news briefing.
Zelensky took office in 2019 on promises of far-reaching reforms but has been criticised for failing to deliver. He told reporters he believed that only 30-35% of his campaign promises had been fulfilled, adding "I do want to do what I promised".
Zelensky denied claims that a president in Ukraine wielded all the executive power. He said he would not seek to change the constitution to replace the mixed form of government.
"I do not plan to turn it into a presidential republic", he said.
Ukraine has a semi-presidential form of government, in which the president and the parliament share the executive power. Zelensky has been criticised for moving Ukraine toward an executive presidency.