Story highlights The European Council formally adopts further sanctions against Russia on Monday

Ukraine and rebels confirm a prisoner release under the Minsk truce agreement

The Kremlin says the Ukrainian and Russian presidents discussed the truce Monday

Ukrainian President Petro Poroshenko confirms death of one soldier, one civilian

Twenty Ukrainian prisoners of war have been released under the ceasefire between Kiev and pro-Russian rebels signed in Minsk, Belarus, last week, Ukraine's President Petro Poroshenko said Monday.

Ukraine's counterterrorist operation press office said the freed prisoners were Ukrainian troops, telling CNN they had been exchanged for a "terrorist."

Speaking in the port city of Mariupol, south of Donetsk, Poroshenko said 33 Ukrainian servicemen who had refused to surrender had also been able to leave the conflict zone.

The Russian state news agency ITAR-Tass quoted the vice premier of the self-proclaimed Donetsk People's Republic , Andrei Purgin, as saying both sides had agreed to exchange "30 for 30" prisoners as a starting point.

Russia's RIA Novosti government news agency quoted DPR Prime Minister Alexander Zakharchenko as saying 28 servicemen had been handed over Sunday.

"It (the prisoner swap) takes place every day. Yesterday we handed over 28 captives and received even more in return," it quoted him as saying.

Ukrainian National Security and Defense Council spokesman Col. Andriy Lysenko said Ukraine had determined the whereabouts of more than 100 troops and efforts were being made to bring them home. He said the exact number of hostages was unknown.

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'We know our enemy well'

The fledgling truce between the Ukrainian government and pro-Russian rebels began Friday evening, though both sides were already accusing each other of violating it by Saturday.

Lysenko accused militants of trying to provoke Ukrainian forces in order to blame Kiev for violations.

Poroshenko expressed hope the truce would hold.

"I believe that the efforts of diplomats will be successful," he told military and border forces. But the Ukrainian leader warned that the country should not be caught off guard: "We know our enemy well. We know the price of his words. I've got no illusions about our opponent, but I've got hope."

The Kremlin issued a statement saying that Russian President Vladimir Putin had spoken by phone with Poroshenko on Monday. "The leaders continued to discuss steps to facilitate the peaceful settlement of the situation in southeast Ukraine. The dialogue will be continued," it said.

In spite of the ceasefire agreement, Putin remains under pressure from the West over Russia's actions in Ukraine.

The Ukrainian government and the West accuse Moscow of supporting the rebels with weapons and fighters -- allegations that Moscow has repeatedly denied.

EU announces new sanctions

Hours after the truce went into effect, European Union leaders meeting in Belgium agreed on a new round of economic sanctions against Russia.

"The sanctions aim at promoting a change of course in Russia's actions destabilising eastern Ukraine and come in the wake of the European Council of 30 August, which condemned the increasing inflows of fighters and weapons from the territory of the Russian Federation into Eastern Ukraine and the aggression by Russian armed forces on Ukrainian soil," he said in a statement.

The measures will take force once they have been published in the Official Journal of the European Union.

"This will leave time for an assessment of the implementation of the cease-fire agreement and the peace plan. Depending on the situation on the ground, the EU stands ready to review the agreed sanctions in whole or in part," the statement said.

"The EU would be better off supporting Donbas' economic revival and bringing normalcy back to the region instead of frantically seeking to put more strain on Russian and its own economies," it said. Donbas refers to a southeastern region of Ukraine encompassing Donetsk and Luhansk.

Mariupol

Poroshenko told staff at a metal works in Mariupol that Ukrainian forces would fight to ensure the city remained part of Ukraine.

"As commander in chief, I have commanded to ensure the defense of Mariupol and the regime of high combat readiness. Ukrainian militaries managed to stop tank attacks on Mariupol by which the enemy tried to ensure the passage to the Crimea," he said.

Mariupol is a strategic port city, lying between the Russian mainland and Crimea, which Russia annexed earlier this year.

Poroshenko said that Ukraine needed financial and technical assistance and was confident that enhanced cooperation with NATO would help facilitate that.

"NATO managed to reach an agreement with several countries on direct supply of state-of-the-art weapons that will help us defend ourselves and win."

Ceasefire deaths

Sporadic artillery and machine gun fire rang out in the early hours of Sunday on the outskirts of Mariupol. A gas station was set on fire, and cars carried wounded civilians down the roads.

Shelling and explosions were also heard near the airport of the flashpoint city of Donetsk on Sunday morning, the city council said on its website.

In both cities, the source of the weapons fire wasn't immediately clear.

The Mariupol city government said that pro-Russian rebels had carried out the shelling near the city overnight. It said one woman was killed and three people wounded.

Poroshenko on Monday confirmed that one civilian had died in the city, as well as a Ukrainian soldier in Stanytsia-Luhanska.

ITAR-Tass late Saturday quoted rebel officials as saying that Ukrainian forces continued to shell Donetsk and rebel positions near Mariupol. RIA Novosti reported that four Donetsk residents were killed in the shelling.

'So much confusion'

Ukrainian forces and pro-Russian rebels have been locked in vicious fighting in eastern Ukraine since April, leaving more than 2,200 people dead, according to the United Nations.

The conflict has triggered a humanitarian crisis in the region, where shelling has destroyed homes and infrastructure. Nationwide, more than a million people have been displaced from their homes by the fighting, most of them in the East.

Ukrainian forces and the rebels have engaged in fierce fighting over the past week in the territory between the Russian border and Mariupol.

A previous unilateral ceasefire declared by the Ukrainian government in June broke down after 10 days.

Amnesty International, meanwhile, accused Ukrainian militias and rebels of carrying out war crimes . The human rights group singled out Russia for a buildup of armor and artillery in eastern Ukraine.

Putin has voiced sympathy for the rebels, many of whom are ethnic Russians. But he denies that Russia has armed and trained the rebels, or sent its troops over the border.