"There is a threat today that there will be war tomorrow," Zelensky told a news conference. "We are entirely prepared for an escalation."
"We need to depend on ourselves, on our army. It is powerful," he added.
Western countries have this month raised alarm over reported Russian military activity near Ukraine.
Moscow has dismissed growing Western claims that Russia may be preparing to invade Ukraine, accusing NATO of provocations.
Earlier this week, Ukrainian Foreign Minister Dmytro Kuleba warned Russia that it will "pay dearly" for an invasion.
The Ukrainian army has come a long way since 2014, when it was caught off guard by Russia's annexation of Crimea.
Ukraine and its Western allies also accused Russia of sending troops and arms across the border to support separatists who carved out two statelets in the east, but Moscow has denied those claims.
Kiev's soldiers have since accumulated combat experience and received arms and hardware from Western allies, particularly the United States which has committed $2.5 billion in support of Ukraine's forces since 2014.
The conflict in the east has claimed more than 13,000 lives.
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