Speaking during a military parade, President Petro Poroshenko said Ukraine must not be complacent even though hostilities have largely died down.
In a show of force, thousands of Ukrainian servicemen marched in downtown Kiev to commemorate the country's independence from the Soviet Union on Aug. 24, 1991.
"We stand for peace, but we are not pacifists," Poroshenko said.
"We must get through the 25th year of independence as if on brittle ice. We must understand that the smallest misstep could be fatal. The war for independence is still ongoing."
Poroshenko didn't say how many more troops he would send to eastern Ukraine.
He said that Russia had massed about 50,000 troops on the border with Ukraine, still had 9,000 soldiers in eastern Ukraine and had supplied the rebels with about 500 tanks and 400 pieces of artillery.
Poroshenko warned that Russia is wary of carrying out an outright invasion and is instead developing another strategy: sow discord across all of Ukraine and thus spoil its relations with its Western allies.
Poroshenko compared the rebel-held territories in the east and their viability to the evil kingdom of Mordor from J.R.R. Tolkien's "Lord of the Rings" novels.
Russia's foreign minister said Poroshenko's statements about Russian troops were "unsubstantiated and unscrupulous."
"It is difficult to escape the thought that their goal was to break the genetic code that guarantees the unity of our peoples," Sergey Lavrov said.
"I don't think he will succeed."
The Ukrainian troops taking part in the Independence Day commemorations carried rifles, but, unlike last year, the parade didn't feature any of the more powerful weaponry.
This could have been seen as provocative because of the conflict in the east, which has claimed more than 6,900 lives since it began in April 2014 and saw a major uptick in violence last week with nine civilians and soldiers killed in just one day.
Ukraine's military said Monday that the rebels violated a cease-fire 82 times overnight in the eastern part of the country, in some cases with large-caliber weapons that should have been withdrawn in line with a truce signed in February.
Poroshenko later traveled to Berlin for an evening meeting with the leaders of France and Germany, in which the three reasserted that the ceasefire agreed to in Minsk, Belarus in February must be implemented.
German Chancellor Angela Merkel said there are still hostilities and observers for the Organization for Security and Cooperation in Europe observers cannot move freely to assess the situation.
"Everything possible must be done to ensure that the ceasefire is a reality," she said.
Poroshenko said that Ukraine is fulfilling its obligations on the ceasefire, the withdrawal of heavy weapons and on humanitarian issues.
"We clearly declare that today Russia and the fighters it supports are the only threat to the peace process. "
Moscow did not send a representative to Berlin, but said it would watch the meeting closely, and Merkel said that there was still regular contact with Russia.
A top French official, who spoke on condition of anonymity because he wasn't allowed to speak on the issue, said the gathering was planned as a three-party meeting and that talks also including Russia could be expected "in the next few weeks."
Poroshenko told reporters in Kiev that the meeting was crucial for Ukraine, Germany and France to "coordinate their positions" before a possible meeting with Russian President Vladimir Putin.
On front-line positions in eastern Ukraine, the mood was less festive.