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.
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.
Members of a self-defence unit from the Maidan protests construct a "corridor of shame" at the entrance to parliament made up of portraits of Ukrainian deputies from the Party of Regions and Communist Party laid out on the ground outside the assembly in K
Russia cut gas supplies to Ukraine as a payment deadline passed and negotiators failed to reach a deal on gas prices and unpaid bills amid continued fighting in eastern Ukraine, June 16, 2014.
Pro-Russian fighters walk past remnants of a downed Ukrainian army aircraft Il-76 at the airport near Luhansk, Ukraine, June 14, 2014.
A man shows a piece of used weaponry at the site of fighting in Maruipol, Ukraine, June 13, 2014.
A policeman uses a mobile phone near an anti-tank mine at the site of fighting in the eastern Ukrainian port city of Mariupol, June 13, 2014.
Ukrainian troops riding on the back of a truck gesture to local residents gathered near the site of fighting, Mariupol, June 13, 2014.
Local residents observe and photograph destroyed vehicles, Mariupol, June 13, 2014.
Residents look at a destroyed armored personnel carrier of the DNR (Donetsk People's Reublic), Mariupol, June 13, 2014.
A home-made explosive is seen on top of a damaged automobile at the site of fighting in the eastern Ukrainian port city of Mariupol, June 13, 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.