Tensions between Russia and the West could spike further on Tuesday when Dutch investigators release a preliminary report into the downing of a Malaysia Airlines jet over eastern Ukraine on July 17. Nearly 300 people died in the tragedy, for which the United States and Ukraine have blamed pro-Russian rebels armed with surface-to-air missiles provided by Russia. Moscow has blamed Ukraine.

In several parts of eastern Ukraine on Monday, there were reports of sporadic violations of the cease-fire. Although the fighting between pro-Russian separatists and government troops has quieted substantially since the cease-fire was announced on Friday, it has not stopped. Still, officials on all sides seemed eager to sustain the truce — by force of will if nothing else.

During his visit to Mariupol, an industrial port city on the Sea of Azov, Mr. Poroshenko spoke by telephone with President Vladimir V. Putin of Russia, and their offices each issued brief statements saying the two leaders were working to support the cease-fire.

The Kremlin said the two leaders “continued discussion of steps to facilitate the peaceful settlement of the situation in the southeast of Ukraine,” and added, “Dialogue will continue.” Mr. Poroshenko’s office said that he had “stressed the importance of effective monitoring” of the truce by the Organization for Security and Cooperation in Europe.

A 12-point agreement reached last week in Minsk by representatives of the Ukrainian government, the separatists, Russia and the security organization includes a proposal to increase political autonomy in eastern Ukraine, but a long-term political settlement is expected to be extremely difficult to reach.

In a speech in Mariupol, Mr. Poroshenko said the sides were complying with a provision in the truce agreement that calls for the exchange of all captives, Ukrainian and international news agencies reported. He said the separatists had released 1,200 captives to the Ukrainian side so far.

Image Mr. Poroshenko with army troops at the Mariupol airport on Monday. Credit... Pool photo by Mykola Lazarenko

In Kiev, the Ukrainian capital, a military spokesman said the cease-fire was largely holding, even though there were repeated violations scattered across the southeast that he called “provocations.”