Tensions over Ukraine festered after Kiev accused the Kremlin of seeking to "eliminate" the pro-Western former Soviet nation while Moscow charged Washington with orchestrating the entire crisis.

The bitter exchange on Saturday in the wake of the toughest Western sanctions yet on Russia came with a fragile nine-day truce once again tested by an hourslong battle for control of a strategic eastern Ukrainian airport.

Ukrainian Prime Minister Arseniy Yatsenyuk called on world leaders not to trust Russian President Vladimir Putin despite his decision to sign Moscow up to a truce aimed at ending a five-month war that has claimed more than 2,700 lives.

The European-mediated peace deal that Kiev signed with Moscow and two rebel leaders has helped calm fighting across the economically vital but devastated industrial rust belt that hugs Russia's border in eastern Ukraine.

But both the United States and Europe remain deeply suspicious of Putin's intentions and are still waiting for him to pull back 1,000 paratroopers they claim have helped insurgents claw back territory in the days preceding the truce.

Moscow not only denies backing the fighters but also accuses Washington of fomenting the February protests that ousted a pro-Kremlin leader and brought in a new team that struck a historic EU alliance and is now seeking NATO membership.

Russian Foreign Minister Sergei Lavrov took particular exception to measures that tightened existing US sanctions and that, for the first time, target two private oil companies as well as the natural-gas giant Gazprom.

Lavrov accused Washington of "trying to use the crisis in Ukraine to break economic ties between the EU and Russia and force Europe to buy US gas at much higher prices".

Russia supplies about a third of Europe's natural gas needs - a reliance that forced Brussels to shield Gazprom from its other sanctions on state-held energy firms.

But Washington added Gazprom to the list of energy firms that will be denied access to advanced exploration equipment needed for new projects.

Top Russian banks and energy companies have also been barred from borrowing from both US and European capital markets for longer than a month.

The punitive steps by the US further target a giant defense firm run by a modern-day billionaire who served alongside Putin in the Soviet spy agency in the 1980s.

"These sanctions will get Putin's attention," the Eurasia Group political risk consultancy said.

But both US and EU officials have promised to scale back or cancel the measures if Russia sticks by the cease-fire deal.

A convoy of more than 200 white trucks crossed the border with Russia to deliver humanitarian aid to a battered Ukrainian city on Saturday, a move made without Kiev's consent but met with silence by Ukraine's top leaders.

"Early in the morning, we entered Ukraine to bring aid to Luhansk," said Yury Stepanov, a Russian who was overseeing the convoy. "We came in around 215 vehicles," he said, as workers unloaded boxes into a local warehouse.

The much-needed aid arrived as fighting flared again between rebels and government forces, further imperiling an already fragile cease-fire.

AFP - AP