Ukrainian President Petro Poroshenko has said it is impossible to win the conflict in the east by force alone, but also warned that his military would deal a "crushing defeat" to pro-Russian rebels if they tried to advance in violation of the ceasefire.
On a surprise visit to the flashpoint city of Mariupol on Monday, Poroshenko also called for the removal of Russian troops.
"It is impossible to win the conflict just by military means," he said, adding that the quickest way to achieve "stability and peace" in Ukraine was simple.
"Withdraw foreign troops and close the border and within a week we [will] find a compromise... I think the peace initiative is bringing us to this result," he said.
Sporadic fighting has flared between government forces and pro-Russian rebels in Mariupol despite a ceasefire agreed on Friday, and the president warned the rebels of defeat should they make a push on Mariupol.
"I have ordered [the military] to secure the defence of Mariupol with howitzers, multiple rocket launchers, tanks,
anti-tank weapons and air cover," Poroshenko told a crowd of steel workers in the port on the Sea of Azov near the Russian border.
Earlier on Monday, Poroshenko said that 1,200 Ukrainian captives had been released by rebels, although it was not immediately clear if this was part of the ceasefire backed by Kiev and Moscow that was signed in Minsk on Friday.
He said there had been 10-12 violations of the truce each day but that if the accord was bringing home prisoners of war and saving the lives of civilians and soldiers "this means the ceasefire works".
The ceasefire, which took effect on Friday evening, is part of a peace plan intended to end a five-month conflict the United Nations estimates has killed about 2,600 people.
Earlier, Ukrainian military spokesman Andriy Lysenko told a news briefing in Kiev that Ukrainian forces were observing the ceasefire except in self-defence and had remained in their positions since Friday evening.
In rebel-held Donetsk, eastern Ukraine's industrial hub, Prime Minister Alexander Zakharchenko of the self-proclaimed
Donetsk People's Republic fired similar accusations against the Ukrainian side, adding: "We have shown the whole world we are not terrorists, we are ready for talks and we can listen."
Kiev and its Western backers accuse Russia of sending troops across the border and arming the rebels, charges Moscow denies.
The conflict has caused the sharpest confrontation between Russia and the West since the Cold War.