Ukraine's president took advantage of a fragile cease-fire to tour a once-embattled port city Monday and tell backers that his forces would never yield the territory to pro-Russian rebels.

The visit to Mariupol was strong on symbolism as President Petro Poroshenko addressed workers at a metal plant from a stage decorated with Ukraine's yellow-and-blue colors. But it also served as a message to the rebels -- and their supporters in Moscow -- that Ukraine's military was prepared to defend the strategic area if the truce imposed last week unravels.

"This city is and will be Ukranian,'' Poroshenko told hundreds of workers, according to the presidential website.

Mariupol, which came under rebel shelling last week, is a key prize in the territorial showdowns in Ukraine. It occupies a strip connecting the mainland with Crimea, the Black Sea peninsula annexed by Russia in March. Mariupol's fall to rebel control could cost Ukraine more coastline and give separatists important new footholds in eastern Ukraine.

Ukraine's military said the truce, which took effect Friday, was generally holding. It has allowed both sides to exchange prisoners. Poroshenko said rebel forces released 20 Ukrainian soldiers Monday.

The break in hostilities came as government forces and rebels regrouped after weeks of increasingly bloody fighting. Divisions about the political future of the country remain as deep as ever, however, fueling skepticism about whether the cease-fire can endure.

Also Monday, the European Union formally adopted new sanctions against Russia over its role in the conflict but said the enforcement would be delayed to assess the implementation of the truce agreement, the Associated Press reported.

Russian Prime Minister Dmitry Medvedev earlier told the Vedomosti newspaper that his country would take retaliatory action if the E.U. adopts new punitive measures. One option, he said, would be to ban E.U. airlines from flying over Russian airspace in transit to and from Asia.

Speaking to reporters in Kiev, Ukrainian military spokesman Andriy Lysenko said Monday that there were instances of cease-fire violations in the restive east, including shelling near the train station in Donetsk. However, he insisted that the truce was holding in general and described the attacks as sporadic and largely free of higher-grade weaponry.

Lysenko declined to offer details about a claim by a senior Ukrainian official on Sunday that five nations -- the United States, Italy, Norway, Poland and France -- had offered to supply weapons to Ukraine. Officials from all the countries, except for France, denied such a deal.

"As soon as we get any technical assistance, we will inform you," Lysenko said.

At the same time, Poroshenko faced the growing challenge of selling the cease-fire pact to Ukrainians. Some critics believe that the government, confronting overwhelming force in the east, had broadly accepted Russian terms and that eastern regions could be left under Moscow's influence.

The deal came together last week after a major offensive by the rebels. NATO and Ukrainian officials say the rebels are being aided not only by Russian arms but also by Russian troops -- charges that Moscow denies.

The United States, Ukraine and other nations are conducting joint military exercises in the Black Sea this week, and Washington has pledged $60 million in nonlethal assistance to Kiev. But the Obama administration has drawn the line at lethal assistance, and one U.S. official familiar with the situation said Sunday that despite Ukrainian claims, "our position remains the same."

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Birnbaum reported from Moscow.