Fighting flared near an airport in eastern Ukraine on Saturday in breach of a fragile eight-day ceasefire as the prime minister accused Russian President Vladimir Putin of planning to destroy his country.
Prime Minister Arseny Yatseniuk said only membership of NATO would enable Ukraine to defend itself from external aggression.
Kiev and its Western backers accuse Moscow of sending troops and tanks into eastern Ukraine in support of pro-Russian separatists battling Ukrainian forces in a conflict that has killed more than 3,000 people. Russia denies the accusations.
A ceasefire negotiated by envoys from Ukraine, Russia, the separatists and Europe's OSCE security watchdog, has been in place in eastern Ukraine since Sep. 5 and is broadly holding despite regular but sporadic violations, especially in key flashpoints such as Donetsk.
On Saturday afternoon, a Reuters reporter heard heavy artillery fire in northern districts of Donetsk, the largest city of the region with a pre-war population of about one million. He saw plumes of black smoke above the airport, which is in government hands. The city is controlled by the rebels.
Speaking at a conference in Kiev attended by Ukrainian and European lawmakers and business leaders on Saturday, Yatseniuk made clear he did not view the ceasefire as the start of a sustainable peace process because of Putin's ambitions.
"We are still in a stage of war and the key aggressor is the Russian Federation ... Putin wants another frozen conflict (in eastern Ukraine)," said Yatseniuk, a longtime fierce critic of Moscow and a supporter of Ukraine's eventual NATO membership.
Yatseniuk said Putin would not be content only with Crimea — annexed by Moscow in March — and with Ukraine's mainly Russian-speaking eastern region.
"His goal is to take all of Ukraine ... Russia is a threat to the global order and to the security of the whole of Europe."
Ukrainian military spokesman Andriy Lysenko told a daily briefing that one soldier and 12 rebels had been killed in the past 24 hours, without specifying where they had died. That would bring the death toll among Ukrainian forces since the start of the ceasefire eight days ago to six.
The rebels have not said how many of their men have died in the same period.
Government forces still hold Donetsk airport, while the city is in separatist hands.
Putin says Russia has the right to defend its ethnic kin beyond its borders, though Moscow denies arming the rebels and helped broker the current ceasefire with Kiev.
Asked about future NATO membership, a red line for Russia, Yatseniuk said he realized the alliance was not ready now to admit Kiev, but added: "NATO in these particular circumstances is the only vehicle to protect Ukraine."
There is no prospect of the Atlantic alliance admitting Ukraine, a sprawling country of 45 million people between central Europe and Russia, but Kiev has stepped up cooperation with NATO in a range of areas and has pressed member states to sell it weapons to help defeat the separatists.
On Saturday, about 100 Russian lorries arrived in the war-ravaged eastern city of Luhansk, part of a convoy sent to deliver 1,800 tonnes of humanitarian aid to residents.
It is the second such Russian aid convoy and it passed the border without any major difficulty. The first convoy in August was denounced by Ukraine and its Western allies for crossing the border without Kiev's permission.
The Ukraine conflict has triggered several waves of Western sanctions against Russia, most recently on Friday. The new measures, branded by Putin "a bit strange" in view of the ceasefire, target banks and oil companies.
Russia, which has already introduced bans on a range of U.S. and European food imports, signalled it would respond with further sanctions of its own against Western interests.
Yatseniuk said on Saturday the latest sanctions posed a big threat to the Russian economy.
"It is a bluff (by Russia) to say it does not care about the sanctions," he said, noting that Russia relied heavily on its energy sector and some of the sanctions targeted its oil firms.
Yatseniuk defended his government's efforts, despite the conflict, to tackle rampant corruption and overhaul the creaking economy, adding: "It is very hard to attract investors when you have Russian tanks and artillery in your country."
His center-right People's Front party is expected to do well in a parliamentary election on Oct. 26.
The conflict is taking a heavy toll on Ukraine's already battered economy, which is now being supported by a 17 billion dollar loan package from the International Monetary Fund.
The economy could shrink by as much as 10 percent this year, the head of Ukraine's central bank, Valeria Hontareva, was quoted by Interfax news agency as saying on Saturday, much more than the 6.5 percent decrease previously forecast by the IMF.
Yatseniuk praised a decision on Friday to delay the implementation of a new trade pact with the European Union until the end of 2015. He said it prolonged unilateral trade benefits now enjoyed by Ukrainian firms in the EU while maintaining modest customs duties on European products entering Ukraine.
Some have seen the decision to postpone the implementation of the deal as a diplomatic victory for Russia, which is opposed to closer economic ties between Kiev and the EU, but Yatseniuk said it would be good for Ukraine's own economy.
"We got a grace period. The EU opened its markets but Ukraine is still protected, so for Ukraine this is not a bad deal," he said.
Deputy Foreign Minister Danylo Lubkivsky submitted his resignation, saying: "(The delay) sends the wrong signal — to the aggressor, to our allies and, above all, to Ukrainian citizens."