Ukrainian President Petro Poroshenko visited the eastern port city of Mariupol on Monday and attempted to reassure his compatriots by promising a “crushing defeat” to pro-Kremlin rebels gathered nearby if they violated a precarious cease-fire agreement.

Poroshenko received a warm welcome and declared that the rebels had returned over 1,200 prisoners of war to Ukraine as per the cease-fire terms, Reuters reports.

“I have ordered [my armed forces] to secure the defense of Mariupol with howitzers, multiple rocket launchers, tanks, antitank weapons and air cover,” he told the gathered crowd.

The cease-fire, which began on Friday evening, has largely held and provided some respite in a five-month conflict that the U.N. says has killed more than 3,000 people. However, reports say that a violation took place Saturday night on the outskirts of Mariupol, where one woman was killed and four others injured when government forces encountered prolonged shelling.

On Tuesday, news agency Interfax reported that four Ukrainian soldiers have been killed and 29 injured since the cease-fire began.

The industrial center of Donetsk, which remains under the control of rebels, has also seen sporadic shelling and gunfire, Reuters reports.

The Organization for Security and Cooperation in Europe, a group overseeing the cease-fire, said the truce itself is insufficient and urged both sides to seek a “political breakthrough.”

Poroshenko represents the Ukrainian government in Kiev, whose authority the self-proclaimed Donetsk People’s Republic rebel faction is refusing to recognize. The Ukrainian government has accused Moscow of arming the rebels and sending troops across the border, but Kiev’s military spokesperson Andriy Lysenko admitted Monday that there had been no visible troop movement for at least 24 hours.

Write to Rishi Iyengar at rishi.iyengar@timeasia.com.