Ukrainian President Petro Poroshenko has said implementing the terms of a new peace deal that was reached after marathon negotiations in Minsk will prove to be "not easy."
Speaking from Brussels late February 12, Poroshenko described his 16 hours of talks with Russian President Vladimir Putin, French President Francois Hollande, and German Chancellor Angela Merkel as "very difficult."
The talks that started on February 11 and concluded the following day were the latest attempt to stop the fighting between Ukrainian government forces and pro-Russian separatists that has claimed nearly 6,000 lives in 10 months.
In Washington, U.S. Secretary of State John Kerry said, "We will judge the commitment of Russia and the separatists by their actions, not their words."
Kerry said the United States is "prepared to consider rolling back sanctions on Russia when [the agreements] are fully implemented."
Poroshenko, Putin, Merkel, and Hollande signed off on a road map for a Ukrainian peace deal after the overnight negotiations.
Terms Of The Cease-Fire
Poroshenko and Putin said the agreement calls for a cease-fire to begin on February 15, for heavy weapons to be withdrawn, and a security zone to be established.
WATCH: Poroshenko Says Agreement Reached On Cease-Fire
The agreement was reportedly also signed by the self-proclaimed leaders of the pro-Russian separatists in Ukraine's Luhansk and Donetsk regions.
Merkel and Hollande said Putin had pressured the rebel leaders, Igor Plotnitsky and Aleksandr Zakharchenko, to sign the deal.
Poroshenko said the document does not grant autonomy to the regions held by separatists and also envisages Ukraine to reestablish full control over its border with Russia by the end of 2015.
He added that Kyiv did not agree to Russian demands for the federalization of Ukraine.
The agreement also calls for the withdrawal of all foreign forces, equipment, and mercenaries from Ukrainian territory.
Poroshenko also noted the deal calls for the release of all people held captive by both sides within three weeks on an "all-for-all" basis and that it should be completed no later than five days after the withdrawal of heavy weapons from the line of contact.
Poroshenko said he was "informed" that Ukrainian Air Force pilot Nadia Savchenko would be released "in the near future" from a Russian jail, where she has been held for more than six months on charges of involvement in the killing of two Russian journalists in Ukraine.
But Mark Feigin, a lawyer for Savchenko in Moscow, said he does not think she will be exchanged with other captives as envisaged by the peace deal but that her trial would be held.
WATCH: Putin Welcomes Minsk Deal
But another Savchenko lawyer, Ilya Novikov, said he hoped she would be released by February 19.
The agreement reached on February 12 is to replace the Minsk agreement reached in September that had been marginally followed, with cease-fire violations considered normal most days as separatist fighters acquired more than 500 kilometers of territory from Ukrainian forces since the deal was signed.
Hundreds of soldiers were also killed and wounded on both sides during the first cease-fire deal.
Cartoons: Lampooning The Minsk Cease-Fire Talks
Separately, a parallel meeting of the three-way "contact group" met comprising representatives from Russia, Ukraine, and the Organization for Security and Cooperation in Europe (OSCE) as well as from the separatist groups. It was led by OSCE special representative Heidi Tagliavini.
The OSCE was tapped to monitor the implementation of the cease-fire and the withdrawal of heavy weapons from the line of contact.
The exact details of where the line of contact would be recognized and whether it would reflect territorial gains made by the rebels in the past six months were not immediately clear.