By PETER LEONARD and LAURA MILLS, Associated Press

MARIUPOL, Ukraine (AP) — Seeking to rally national unity, President Petro Poroshenko visited a southeastern port Monday that has been assaulted for days by Russian-backed separatists and declared the city would remain a part of Ukraine.

After a series of military defeats to increasingly confident rebel forces in the country's eastern regions, Ukraine signed a cease-fire deal Friday that has been widely viewed at home as an act of capitulation. Much of the region has remained calm as the truce appeared to be holding, although sporadic unrest was reported.

"We will do everything to ensure there is peace, but we will also brace ourselves for the defense our country," Poroshenko told metal workers at a plant that was within the range of the rebels' rockets.

Also Monday, the Kremlin said Poroshenko and Russian President Vladimir Putin spoke by telephone and "continued to discuss steps helping peaceful settlement in southeastern Ukraine."

The two leaders had also talked Saturday about implementing the cease-fire plan in the conflict that has lasted nearly five months and killed at least 3,000, according to a U.N. estimate. The fighting has also forced hundreds of thousands to flee their homes.

"The dialogue will continue," the Kremlin said without elaborating.

Also Monday, the European Council formally adopted a package of further sanctions against Russia but delayed enforcement to see if the cease-fire holds.

Council President Herman Van Rompuy said the sanctions will be implemented in the next few days, which would leave time for "an assessment of the implementation of the cease-fire agreement and the peace plan."

The European Union sanctions are expected to be coordinated with a new round of U.S. sanctions, a Western diplomat said. The U.S. sanctions are ready for release, the diplomat said, but the Obama administration wants to wait to act in concert with Europe to maximize the impact of the sanctions and present a united front against Russia. The diplomat was not authorized to discuss the details of the sanctions before they were formally announced and insisted on anonymity.

Poroshenko's trip to Mariupol came days after it came under sustained shelling from rebels stationed along the 70-kilometer (40-mile) stretch between the strategic port on the Sea of Azov and the Russian border.

"This city was, is, and will be Ukrainian!" he told the crowd under the gaze of a tight security detail.

The agreement reached in Minsk, the capital of Belarus, called for an immediate halt to fighting and an exchange of prisoners. It also called for the central government to give a greater degree of autonomy to the separatist Russian-speaking regions of Donetsk and Luhansk, although details remained vague.

While insisting the agreement would not lead to the breakup of Ukraine, Poroshenko expressed some openness to further discussions on the shape of the country.

"I think that we should elect the real representatives of Donetsk and Luhansk. And we, with them, can negotiate anything, excluding the sovereignty of territorial integrity," he said, speaking in English at a news conference in Mariupol.

Poroshenko peppered his conciliatory messages with a gritty determination to repel what he strongly implied was Russia's aggression.

"Mariupol has proven that we won't let anybody burn our city to the ground. The workers of Mariupol have protected peace and calm in the city," he said.

Poroshenko also said thousands of prisoners would be released by the rebels. He later announced that 20 Ukrainian soldiers had been freed.

Ukraine's insistence that Russia is effectively waging a war by proxy has been supported in statements from Western leaders and NATO.

The rebel offensive was bolstered by a substantial resupply of powerful weaponry in the second half of August. Moscow has steadfastly denied it provided that materiel, although regular convoys were seen coming from the direction of Russia by Associated Press reporters around that time.

The area around Mariupol had remained relatively untouched by unrest until the last two weeks, when the city's outskirts were shelled. The attacks raised fears the rebels could link Russia with Crimea, the Black Sea peninsula annexed by Moscow in March. The move would cost Ukraine another huge chunk of its coast and all the mineral riches believed to be under the Sea of Azov.

No casualties were reported overnight Monday in Donetsk, the largest rebel-held city, officials said, although residents reported hearing sporadic explosions later in the day. In Luhansk, another rebel-controlled city hit hard by shelling, authorities reported no fighting for the third night in a row.

Col. Andriy Lysenko, a spokesman for Ukraine's National Security Council, said no serviceman had been killed in the past day. Rebels had stopped using heavy artillery and were only using mortar and rifle fire, he said.

"That's a big achievement," he said.

Still, he said the rebels had violated the cease-fire a half-dozen times.

While Poroshenko told journalists in Mariupol that Ukraine's "territorial integrity" was not a negotiating point, rebel commander Alexander Zakharchenko told the Russian radio station Kommersant FM that he would like to see "an acknowledgment of (our) independence" added to the Minsk protocol.

Poroshenko also emphasized that he would only negotiate with "elected leaders" of the region, but who exactly that meant was left unclear. While parliamentary elections are scheduled for Oct. 26, the rebels could easily derail the vote in the eastern regions.

The cease-fire deal has left Poroshenko on the defensive in the rest of Ukraine, and many politicians criticized it as treachery. Yulia Tymoshenko, the former prime minister and the leader of one of the largest political parties, was quoted by the Interfax-Ukraine news agency Monday as saying the Minsk agreement was "extremely dangerous" because it did not demand that Russian forces leave Ukrainian territory.

Poroshenko acknowledged in Mariupol that "some people do not like" the deal.

"Yes, we have enough strength to defend both the city and the country," he said. "But everyone wants peace, and that's why I started talks with President Putin."

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Mills reported from Kiev. Associated Press writers Vladimir Isachenkov in Moscow and Juergen Baetz in Brussels contributed from Moscow.