Ukraine's President Petro Poroshenko says he will put forward a detailed peace plan that will include a ceasefire in the government's battle with pro-Russian separatists.
Opening a meeting the country's national security council in Kyiv, Poroshenko said Monday he would roll out the peace plan this week, and that a ceasefire would be proposed "as the beginning" of the plan's implementation.
He said the plan would include changes to the country's constitution that would de-centralize power - a key demand of many government opponents in eastern Ukraine.
Poroshenko says he has ordered the army to take "decisive actions" this week to regain control of the Ukrainian-Russian frontier.
He also said Ukrainian troops have regained control of a little more than 10 percent of the border, and that checkpoints are in place aimed at stopping the flow of Russian military hardware into his country.
Poroshenko's comments followed an announcement by Russian energy giant Gazprom that it is cutting natural gas supplies to Ukraine after Kyiv missed a deadline to pay nearly $2 billion of its outstanding debt.
Gazprom said Ukraine must now pay in advance for any natural gas.
Also Monday, Gazprom filed a lawsuit in a Stockholm arbitration court to try to recover what it says is Ukraine's entire $4.5 billion debt.
Pro-Russian troops prepare to travel in a tank on a road near the town of Yanakiyevo, Donetsk region, eastern Ukraine, June 20, 2014.
People carry their belongings as they walk to cross the border into Russia at the Ukrainian-Russian border checkpoint in Izvaryne, eastern Ukraine, June 20, 2014.
A man examines a destroyed building after fighting between Ukrainian and pro-Russian fighters in the city of Artyomovsk, Donetsk region, eastern Ukraine, June 20, 2014.
Pro-Russian fighters wave a white flag to start a handover of the bodies of Ukrainian troops killed in a plane shot down near Luhansk, at a check point in the village of Karlivka near Donetsk, eastern Ukraine, June 18, 2014.
Pro-Russian fighters wait for representatives of the Ukrainian troops at a checkpoint in the village of Karlivka for the handover of the bodies of Ukrainian troops who died in a plane shot down near Luhansk, Ukraine, June 18, 2014.
Pro-Russian separatists stand guard at a check-point as a car drives past outside Luhansk, Ukraine, June 18, 2014.
Miners, one of them carrying a sign with the name of the mine Trudovskaya, march in support of peace in Donetsk, eastern Ukraine, June 18, 2014.
Residents collect water at a pumping station in the eastern Ukranian city of Slovyansk, June 17, 2014.
A woman walks past portraits of protesters who were killed in clashes with police in February in Independence Square in Kyiv, June 18, 2014.
People take part in a rally to press demands for parliament to be dissolved and early elections outside the assembly in Kyiv, June 17, 2014.
Pro-Russian fighters walk past remnants of a downed Ukrainian army aircraft Il-76 at the airport near Luhansk, Ukraine, June 14, 2014.
In Kyiv, Ukraine’s prime minister, Arseniy Yatsenyuk, angrily rejected the Russian position. Speaking hours after negotiators failed to reach a gas deal, he accused Moscow of using the issue as part of its larger designs on Ukraine.
“This is not about gas. This is part of Russia’s plan to destroy Ukraine. This is all but the next stage in Russia’s aggression against Ukraine and its independence,”Yatsenyuk said.
Ukraine filed a suit of its own against Gazprom in Stockholm.
Ukraine is a conduit for gas supplies to other parts of Europe. Gazprom notified European countries that shipments through Ukraine will not be affected, but warned there could be a disruption if Ukraine decides to siphon off the gas. A Gazprom spokesman said Ukraine's state-owned Naftogaz is obliged to make sure the gas reaches its European customers.
Europe gets about one-third of its natural gas from Russia, half of which flows through pipelines across Ukraine.
Russia and Ukraine have been embroiled in the energy dispute since April when Moscow nearly doubled the price of gas to Ukraine to $480 per 1,000 cubic meters. Russia has since offered about a 20 percent discount, but Kyiv negotiators have sought a deeper price cut, closer to the original price.
Ukrainian officials say the country has almost 14 billion cubic meters of gas in underground storage, which is enough to meet its needs until December.
Sunday's energy talks began as Ukraine concluded a national day of mourning for 49 Ukrainian service members who were killed when their military transport plane was shot down in eastern Ukraine.
Ukraine's defense ministry said the plane was trying to land in Luhansk early Saturday morning when it was downed by pro-Russian separatists.
Some reporting by Reuters, AP and AFP.