Negotiators in Ukrainian peace talks agreed on early Saturday to create a buffer zone between government troops and pro-Russian militants by halting their advances, pulling back heavy weapons and withdrawing foreign fighters in order to ensure a stable truce in eastern Ukraine.

The deal reached by representatives of Ukraine, Russia, the Moscow-backed rebels and the Organization for Security and Cooperation in Europe marks an effort to add substance to a cease-fire agreement that was signed on Sept. 5 but has been frequently broken by clashes.

The memorandum signed after hours of talks that dragged late into the night says that the conflicting parties should stay strictly where they were on Friday and make no attempts to advance.

Leonid Kuchma, a former Ukrainian president who represented the Kiev government in the talks, said the memorandum will be implemented within a day.

Under the terms of the deal, reached in the Belarusian capital, Minsk, each party must pull back artillery of 100 millimeters or larger at least 15 kilometers, setting up a buffer zone that would be 30 kilometers wide.

The longer-range artillery systems are to be pulled even farther back to make sure the parties can’t reach one another.

The deal also specifically bans flights by combat aircraft over the area of conflict and setting up new minefields.

“It should offer the population a chance to feel secure,” said Igor Plotnitskyi, the leader of rebels in the Luhansk region.

The rebels are located near the cities of Donetsk and Luhansk in eastern Ukraine and the port city of Mariupol on the Sea Azov coast, but their positions elsewhere are not clear. Ukrainian government forces are at the airport in Donetsk but the location of their lines outside of that city is also unclear.

The memorandum also envisages the withdrawal of “all foreign armed units and weapons, as well as militants and mercenaries” a diplomatic reference to Russians fighting alongside the rebels.

Ukraine and the West have accused Russia of fuelling the insurgency in eastern Ukraine with weapons and soldiers. Moscow has denied that, saying that Russians who joined the mutiny did so as private citizens.

Pressed to comment about the agreement on the withdrawal of foreign fighters, Russian Ambassador to Ukraine, Mikhail Zurabov, who represented Moscow in the talks, said that “those whom we call mercenaries are present on both sides.” “This issue needs to be solved, and we will deal with it,” he said, adding that the OSCE would control the pullout.

The cease-fire that was declared on Sept. 5 has been repeatedly violated. On Saturday, Ukrainian national security council spokesman Volodymyr Polyoviy said about 20 rebels had been killed in clashes with Ukrainian forces over the past day, along with one Ukrainian serviceman.

In Donetsk, the largest rebel-held city, strong explosions broke out on Saturday morning at a munitions factory. A local official, Ivan Pirkhodko, said on Ukrainian television that the explosions were triggered by an artillery shell striking the plant, but it was not clear which side fired it.

The agreement reached on Saturday could be a significant step forward if it is adhered to, but negotiators have not yet addressed the most difficult issue the future status of the rebel regions.

The insurgency in the mostly Russian-speaking Donetsk and Luhansk regions in eastern Ukraine flared up after the ouster of Ukraine’s former pro-Russian president in February and Russia’s annexation of Crimea the following month.

In April, the rebels seized government buildings in the two provinces and declared them independent. They fought government troops to a standstill in five months of fighting that have killed more than 3,000 people and devastated the regions that formed Ukraine’s industrial heartland.

The Ukrainian crisis has pushed Russia-West relations to their lowest point since the Cold War. Faced with several rounds of Western sanctions that badly hurt the Russian economy, Russia’s President Vladimir Putin has pushed for a peace deal that would ease Western pressure while protecting Moscow’s interests in Ukraine.

As part of a compromise to end the hostilities, the Ukrainian parliament this week passed a law giving a broad autonomy to the areas controlled by the rebels, including the power to hold local elections and form their own police force.

Alexander Zakharchenko, the leader of rebels in Donetsk, said after the talks that Ukraine and the rebels have conflicting interpretations of the law and the talks should continue.

In Donetsk, the largest rebel-held city in east Ukraine, the separatists held a city-wide cleanup day on Friday, sending prisoners out to help remove the debris that has piled up after months of shelling.

Throughout the cease-fire, periods of peace have been interrupted by intermittent gunfire. The same was true on Friday, when the Donetsk city council said in a statement that one person was killed by shelling during the night. Col. Andriy Lysenko, a spokesman for the National Security and Defence Council, told journalists in Kiev that two servicemen were killed in the past day during the fighting.

The streets were quiet on Friday as the rebels called for a cleanup. In one school that was shelled in late August, four Ukrainian prisoners guarded by armed rebels were sweeping up debris.