The cease-fire between pro-Russian militants and the Ukrainian government took effect at midnight, following fierce fighting over the town of Debaltseve in the hours before the deadline.

Both sides vowed to abide by the accord signed Thursday in the Belarus capital. Fighting ceased in at least six eastern cities after the deadline, according to residents and a military official. Among them was Debaltseve, the railway hub that was a scene of violent clashes Saturday, according to Vladyslav Seleznyov, a spokesman for the Ukrainian military.

The deal brokered by the leaders of Russia, Ukraine, Germany and France marks the latest effort to defuse the almost one-year crisis that’s killed more than 5,400 people and devastated eastern Ukraine. Similar agreements have failed.

As the deadline hit, President Petro Poroshenko said during a televised speech that he ordered a cease-fire along the front lines. He said he hoped the rebels would abide.

“Unfortunately, the peace process is now under high threat,” he said. “The whole world held its breath and is waiting for tomorrow’s dawn.”

Alexander Zakharchenko, head of the self-declared Donetsk Republic, signed a decree ordering his forces to cease hostilities, though they would be allowed to respond to attacks from government troops. Separatist forces in the nearby region of Luhansk also received orders to cease fire, according to Interfax.

At least one violation of the truce occurred. In the Luhansk region, pro-Russian separatists attacked Ukrainian forces with mortar fire about 25 minutes after midnight, according to Valentyn Nalyvaychenko, the head of the Ukraine security service.

Rebels and government forces made the last push Saturday to maximize territory under their control before the respite, as fighting raged around the railway hub of Debaltseve.

Poroshenko warned that Ukraine will impose martial law if peace doesn’t take hold. At least seven Ukrainian soldiers were killed and 23 wounded in the previous 24 hours, with another death reported near Mariupol.

The mounting doubts over the truce’s viability were reflected in Ukraine’s foreign debt. The country’s benchmark dollar-denominated bonds maturing July 2017 closed 2.4 cents lower at 54.2 cents on Friday, ending a seven-day rally. Russian markets showed signs of optimism that the country will be able to avert tougher sanctions for its role in the conflict, with stocks and bonds extending a second week of gains and the ruble strengthening.

Separatist forces, backed by Russian regular troops, pushed to seize more territory before midnight, said Andriy Lysenko, a Ukrainian military spokesman.

Rockets struck the government-held town of Artemivsk, located about 20 miles from the front line, according to local police. It was the second consecutive day of such attacks.

U.S. intelligence and military officials remain convinced that even if the cease-fire takes hold, the separatist offensive will resume at some point and continue at least until Debaltseve and Mariupol are under Russian control.

Debaltseve is a vital link between Donetsk and Luhansk, the officials said Saturday, speaking on the condition of anonymity to discuss intelligence assessments. Mariupol, a port city on the Sea of Azov, is the last major obstacle to opening a land route between Russia and Crimea.

Debaltseve, a pocket of government-held territory inside areas controlled by separatists, was targeted by rebel assaults before the cease-fire, according to Ukraine’s military. The town and its environs were repeatedly attacked by artillery and tanks, with separatists also shelling areas near Mariupol, the National Security and Defense Council said Saturday.

In his televised address, Poroshenko said he wants officials from the Organization for Security and Cooperation in Europe dispatched to Debaltseve immediately to monitor the truce.

Poroshenko on Saturday spoke with U.S. President Barack Obama, as well as with German Chancellor Angela Merkel and French President Francois Hollande, two of the brokers of the Minsk deal. The leaders expressed concern over the showdown in Debaltseve, according to statements on the Ukrainian president’s website. Obama also spoke with Merkel, the White House said.

The rebels demanded the surrender of Ukrainian forces fighting in Debaltseve and said they seized parts of the town and nearby areas.

“Why should we let them out? We are giving them the chance to stay alive, this is our enemy, who came onto our land,” said Eduard Basurin, a Defense Ministry official from the self- declared Donetsk People’s Republic. “Yes, we will carry out our commitments, but if the enemy advances in our direction, they will be repulsed as needed.”

As tensions persisted on the ground, European Union leaders started to draw up further sanctions to prod Russia to enforce the truce. More than 10,000 Russian troops are in the country, Ukrainian Deputy Defense Minister Petro Mekhed told reporters in Kiev.

Ukraine, the U.S., the EU and the North Atlantic Treaty Organization say Russia is supporting the separatists with hardware, cash and troops, accusations the Kremlin denies. Russia says Ukraine is waging war on its own citizens and discriminates against Russian speakers, a majority in the Donetsk and Luhansk regions.

The U.S. is “very concerned” about the continued fighting, particularly in light of additional Russian military assistance to separatists “in the past few days,” U.S. State Department spokeswoman Jen Psaki said Friday. Russia’s military also has air-defense systems deployed near Debaltseve, she said.

Russian Defense Ministry spokesman Igor Konashenkov denied the claim, which he said was as reliable as “reading tea leaves,” Interfax reported.